As I'm sure you are aware, your choices boil down to either tell her before the event, or tell her after the event.
- Tell her before the event
Among the many drawbacks of this option, is that (despite all the evidence) you might be wrong, and you might seem a little foolish for jumping to conclusions. Alternately it might seem as if you expected your girlfriend to throw you a surprise party, and she might feel that she let you down by not having one.
This alone makes it the less tenable option. While you might draw reasonably logical conclusions from the evidence you've seen, you can't really be sure it's going to happen.
On top of this, it spoils the fun for your girlfriend and your guests. They've come expecting to try to surprise you, with the general understanding that you might not be surprised. If you tell her your suspicions in advance, and she tells others that you figured it out, they'll all likely be disappointed that they weren't able to keep you in the dark.
The benefit of telling her in advance is that you don't have to try to keep up the act until the day of the party, and (depending on who else knows you know) you might not have to act surprised. Also, it's possible that your girlfriend will get upset that you deceived her the whole time, which could upset your relationship ... but this is unlikely, and anyway it would be pretty hypocritical for her to get mad that you deceived her about not being deceived by her.
If you feel that you are unable to keep up the act -- which may be likely, if you have a poor poker face -- then a variation is to tell your girlfriend in advance of the party, but advise her not to tell anyone else. That way you both can work together to make everything appear to go smoothly.
- Tell her afterward
With this option, you get to have your party, and everyone gets to have their fun. The only drawbacks are the previously mentioned "mask of ignorance", and that you have to "trick" your girlfriend and your friends. However you can always respond that you suspected but you didn't know, and anyway you didn't want to spoil their fun.
Moreover, while the party itself may not be a surprise, you might be surprised by who actually shows up, or by the exact circumstances. As long as you don't actively look for clues, there should be something about the party that turns out to be pleasantly unexpected.