They'll know.
Normally on GPUs they have either a couple of stickers that rip and have the word "void" across them, or they have painted screws.
In my GTX 260s I've noticed they have the painted screws, which means when you take the card apart the paint (that was wet when teh card was put together) is now all chipped and often comes completely off if you take the card apart more than a couple of times.
There's few ways you can "unvoid" a GPU.. but after saying all this, it all depends on how strict the brand is. I've voided warranties before and had no issue sending it back as they just never checked and sent a replacement straight the way.
Best not to try and unvoid, you could do more damage painting screws or putting stickers all over it yourself