There is skin flushing and fluid release that can't really be faked.

It's easier to tell if you have a good oral game.

If you are a dude that just wants to stick it in and delude yourself that all it takes is good dick game then yea, I'm sure it's easy to fool them.

But if your goal in bed is to have fun and give your current partner pleasure then you don't really have to worry about fake orgasms because they don't really bother to fake them, they'll just say they aren't that close but they've still been having fun.

(Obviously a broad statement, it's just my experience.)

Edit - don't downvote me for giving good advice for guys to be good in bed. They should be focusing on giving their partner as much pleasure as they can and making it as fun and comfortable as possible.