Just use common sense when buying one.
-if it's skinny with a sunken belly then no
-if it was just shipped in from the wild then no
-if you have a small tank then no, these fish constantly eat, so pods are a must
-if you have aggressive wrasse, in a small tank then no, cleaners can become very skittish if harassed.
-and to reiterate, cleaner wrasses due not remove ick!! that along with cleaner shrimps are one of the biggest misconception in the hobby. Even LFS will promote this false relation.