I don't know that you can tell for sure unless you catch them in the act, BUT, they do have a reputation as being fin-nippers.

Dwarf puffers are cute and sneaky...they roll up on another fish real slow and cute like with their waddly swimming style then casually reach over and ..CHOMP...sadly these cute lil guys end up being in a species tank due to their appetite for tasting their tank mates.
They have so much personality tho, its worth it!

So far these guys dont go anywhere near the other fish. Mabey since there is only 2 of them in a tank of much bigger fish, they are intimidated Haha. I dont know.. we'll see. Hopefully they will work out. Everything is okie dokie so far!

There usually more than fin nippers. I've seen them cut larger fish in half. Get ready to move them when they get hungry.

I've been keeping one in a tank full of guppies for a year now. He does get the tails every now and then on the big delta tailed ones and I'm certain he gets the slower fry. I do keep another tank just to supply him with snails. Wouldn't want to keep him with more expensive fish but he works pretty well with the extremely prolific guppies.
Wish I could convince this guy to eat frozen foods but he just doesn't want it unless it's alive. I've heard of lots of others that will eat frozen though

What kind of snails do the puffers need? Those people at the store never told me they needed snails... but Ive been doing some research. Also... will the snails interfere with my suckerfish? By the way.. If I wanted to keep them in a tank of their own ( I have 2 ) how big should it be?

The snails keep the puffers mouth parts trimmed, they have hard beak like lips and they will overgrow if the fish doesnt keep them trimmed by nipping at hard surfaces like..snail shells. Some puffers have more trouble then other with this. Alot of stores will sell you snails or if they have infestations in their tanks from live plants they will scoop up some and give them to you or sell them for a nominal fee or order you some.
Some snails will eat more then green plants and algae and will clean up uneaten food and stuff from the bottom of the tank, it depends on the variety of snail. They will eat the algae discs and sheets made for plecos who have no tank algae to eat.