It is difficult to tell with some sporting superstars whether they are self-driven, wife-driven or agent- driven when they launch themselves towards another part of the galaxy. It probably doesn't matter at this stage of the David Beckham saga; but it will.
There's no such problem when it comes to Tiger Woods. His father might have put him on the path to greatness at an early age, but Tiger took full charge of the propulsion department and remains at the wheel.
The journey is a little bumpy at the moment. His failure in the US Open last week meant that he is no longer the reigning champion of a Major. He tetchily rejected the word "slump", but it's a lull by his standards, and already it is being asked if we credited him with too much potential in a game notorious for pegging back the ambitious.
The prevailing instinct is to trust him to rekindle his self-belief, and it was a co-incidence that within a day or so Beckham was stepping up for examination. He's not in a crisis, he's in clover, but you know that up ahead is a test of resolve and character that won't come much fiercer. The most extraordinary football transfer of all time has contained so many extra dimensions it is easy to forget that sooner or later he'll have to start playing.
When he does, he won't have a world-record fee to justify - he cost Real Madrid less than half the £48 million they paid for Zinedine Zidane - neither will he have to live up to a reputation as massive as those of some who have recently preceded him into the steeply demanding confines of the Bernabeu stadium.
He has the even more difficult task of justifying an unprecedented celebrity status that is based on far more than mere footballing ability. Real have made no secret that a large part of Beckham's appeal to them involved the ancillary profits he can start earning for the club even before he kicks a ball. Marketing and financial experts have been frothing at their calculators while trying to estimate the peripheral benefits that can be milked from the worldwide admiration he has generated for qualities far removed from what he can do with his feet.
But, sooner or later, those feet will have to replace the smooth talking of the Beckham promotion, publicity and profiteering machine and present a totally different set of accounts before the most sneeringly discerning auditors of all.
It will be an enormous test; all the more so if he is not the sole architect of his ambitions. He needs to be his own driving force. He has rapidly to adjust his game in alien surroundings and to a standard measured higher than that of the Premiership and, if he is following an agenda set principally by others, he will find it discouragingly hard, if not impossible.
No matter how great the commercial empire built around his name, it must be remembered that it is founded on football and can flounder on football.
During the past week, the nation has been divided over Beckham's worth as a footballer and his appeal as a human being. There have been some astounding attacks on his lifestyle and demean-our and one or two have even portrayed him as evidence of what a decadent, low-down slob society we have become. I find this remarkable.
As a footballer, Beckham has the odd failing. In fact, he carries enough limitations to make his achievements all the more commendable. He delivers a superbly weighted and accurate ball, works diligently and can alter the course of a game with the power and deceptiveness of his dead-ball kicking.
I've always been impressed with what he has created for himself. He single-handedly got England into last year's World Cup finals with his performance against Greece, and has played a major role in a great Manchester team for the past several years. He has a habit of rising to the big occasion and could respond well to the challenges I outlined earlier, although there are one or two other factors that worry me.
One of the most fascinating aspects of the unfolding story last week was the conjecture over his relationship with his manager at United, Sir Alex Ferguson, and which of them worked harder to achieve his departure. No doubt we'll have to wait until Beckham's autobiography comes out to discover the true story, although it would be no surprise if Sir Alex's true story came out first.
Coming from the same generation as Sir Alex, I could sympathise with his exasperation at the some of the accoutrements of Beckham's life. The player might well be heralding a new breed of footballer, even a new breed of man, but to us old fogeys he came across as football's first Barbie doll, an incentive for all rich young ladies to get a player they can dress every day.
His hairstyles have been regularly changed for surely no other purpose than to keep his pictures in the newspapers, and he is a frontrunner in the fashion to have your face decorated with designer bumfluff.
But he wins high marks for steering clear of drink and gadding about, and is a sincerely dedicated father and husband.
United without Beckham is almost as interesting a prospect as Beckham without United, but what makes the latter the more formidable challenge is the difference in management, and this is one of the most concerning factors about Beckham's footballing transition.
Whereas Ferguson runs a tight, extremely supportive ship at Old Trafford, Vicente del Bosque is a slightly ghostly figure at Real Madrid. He had absolutely nothing to do with the Beckham deal, which was negotiated by the top brass on the board.
Despite the fact that Real have won two European Cups and are about to win the League, Del Bosque has yet to be told if he is needed for next season. Normally, when a transfer of this magnitude takes place it is spawned by the manager, and his reputation is on the line; so much so that it is very much in his interests for the newcomer to succeed.
Del Bosque does not have that obligation. Indeed, it is said that players like Fernando Hierro and Raul have more control than he does over team matters. Even if a new manager is appointed, he will have his own pressures to deal with.
On Friday, Beckham asked for patience while he was settling down. After all, he said, it was the first transfer of his career. Madrid's players were reported not to be overpleased at the publicity aroused by Beckham's arrival when they were trying to focus on today's title clincher against Athletic Bilbao, and I doubt if they'll be keen on allowing him a long probation period.
Beckham has already had to reassure Raul that he doesn't want to take over the Spaniard's No 7 shirt, and has apologised for the disruption his transfer has caused. But the England captain will already be suspecting that there are no favours awaiting him.
A tour of the United States followed by a rapturous reception in the Far East and a transfer to Real Madrid in the middle - these are enriching times for David Beckham. But they are yet to be enviable. I don't think Tiger would swap.