1: Filing is a huge red flag
No one should ever (ever) abrade your nails while applying or removing gel polish. If they pull out a file, scissors, or any other cutting tool, you should hightail it out of there immediately.
2: Look for labels
Ask to see the gel colors your salon has available and look at the labels. Shellac and other accepted gel brands like Gelish and OPI Axxium have labels on every bottle, so if a nail tech pulls out something unmarked to use on your nails, just say no.
3: One brand should rule them all
Don't let anyone mix systems on you. The three brands mentioned above all have different formulations, so if someone just used a Shellac base coat on you, they shouldn't try to put a Gelish polish on over it. They aren't meant to be used together, and it's not clear what the effects on quality and removal might be.
4: Gels don't "soak off"
When you're having your gels removed, your nail tech should never try to soak your hands in a bowl of acetone. Gel polish comes off with specific gel remover wraps or by using acetone-dipped cotton balls and tin foil to wrap nails. It's gentler and it's what gel-trained nail techs know they're supposed to do.
5: Seek out the seal
If you're on the hunt for Shellac, look for the CND seal of approval on the window of your salon. CND actually sends brand ambassadors to salons to ensure that they're using the right methods.