I have faked it a few times. I've always been surprised that people assume men can't. As long as you have some muscle control (ie, can make johnson dance) it's not hard. As for evidence in the condom, well, just depends on how much natural lube you output.
Mostly I try to never fake it and just be upfront. Most women seem fine with it, I've only had one that I can recall being "upset", and several took it as a challenge (which in itself ended up being rather fun). When I do it's usually because I realize it's not going to happen, we've been at it too long and I'm not going to "relax" ... faking it at that point is often just the simplest way to reach a stopping point and allow my mind to do whatever is necessary to allow johnson to relax.
Overall though, I agree ... don't fake it with me and I won't fake it with you and both of us agree not to take it personally when it doesn't happen.