I've been using fake nails since I was really young. Around 5 maybe? Of course, at that time I didn't care if they looked real or not, I just wanted to have long pretty nails that I could paint any color I wanted! :) Sometimes I even bought the ones that were pre'colored which I never see around anymore.
Something I learned within the last couple years though, is if you don't put them on properly, it's a dead giveaway that they're fake.
1. You always want to push them back into and under your cuticle. ALWAYS. It makes them look like they're growing right out of your finger. :)
2. Always file down the sharp edges. Nothing looks more fake than 90 degree angles at the corner of your nails. Also, give the tips of the nails a bit of a rounded edge. Not to rounded, yet definitely not flat.
3. Choose nails that are longer, or the same length as your own nails. I had a problem with this the other day when I was out shopping. I found a cute pair of nails that I thought would be long enough (because I thought my own nails were short, & they weren't), but almost all of my own nails were longer and stuck past the tips. I had to file my own nails down so I could wear them. You have no idea how mad that made me! lol I've been trying to get my nails to grow out for years, and then all of a sudden I have to file them down? grr.
4. When buying clear nails, never use to much glue. Never apply the glue directly in the center of your nail when using these. You'll be able to see the glue and it will be a complete disaster. Instead, apply it around the outer edges. Mainly around the cuticle and the end of your nail. Usually I'd revise from doing this, but in this case I actually advise it.
5. Hold your nails in place for at least 30 seconds to prevent them from moving. Nothing is worse than crooked nails.
6. When buying French manicure nails, make sure you buy the ones that are the closes to your own skin color. don't get them if they are too light, no matter how cute they are. Just keep searching, and you'll find a pair that will look natural! :)
If you follow those tips, you'll be able to pass off any fake nail as your own! :)