In my opinion, it is much easier to recognize an Munchkin GURPS character than a Munchkin D20 character. The reason is pretty simple, the ways that you munchkinize a GURPS character are so different than the way that you build a balanced character that it is quite obvious what is being done.
Since it is easier to detect, there is a lower likelyhood that a single munchkin character will be allowed into a group that disapproves of such things.
In D20, everyone takes feats. Many will multiclass two and (most rarely) three classes. Just quickly looking over the character sheet, it may not be obvious that combining the Rhino Armor with a Barbarian/Cleric of Kord results in the potential for a huge amount of damage on a charge. Reading the Sultan's of Smack thread on these boards would make it more likely you would catch these things, but even then it is fairly easy to miss.
In D20, you are taking normal rules that are usually fine, and combining them in such a way as to optimize one particular facet of the character. There is no one thing you can look for to detect it, you have to look at several things in combination. In my example, it was the multiple strength pumps (Barbarian Rage, Strength Domain power) with a charge and an item that increases damage on a charge that resulted in the optimized character.
In GURPS, most of the ways to munchkinize are much more obvious than that. Look for the one or two things that are way higher than normal and you've probably found it.
One of the classical ways to munchkinize is buying up an attribute to insane levels (17 is really getting up there) and then becoming a 'half-point wonder'. Never spend more than a half point on most skills, depending on your fantastic attribute for everything. This is pretty easy to spot, the combination of very high attribute with an extremely broad skill base.
Another way to munchkinize is to buy the Eidetic Memory advantage. This advantage probably has more people house ruling or forbidding it than the Harm spell does in D20. If GURPS ever does go into another edition, I believe this advantage would be heavily modified. After seeing one character abuse this advantage, most GURPS GMs know to look out for it.
Another way to munchkinize is to buy a single power up to extremely high levels. Become so focused on just that one (or maybe two) things that it becomes a 'juggernaut' ability. In a setting where the ability comes into play, almost nothing can stop you. Where it doesn't come into play, you are useless.
All of these GURPS things are pretty obvious when you look at the character sheet. Look for the high attributes, high skills, or single power that is taken to insane levels. If one of your players has a skill 25 when everyone else is down below skill 15, you're probably looking at a munchkin character.
Note: If every character is expected to have skills of 20-23, a skill of 25 is not unreasonable. It is only when there is a large gap between what the GM thinks the skill values should be and what the character has that there is a problem.