Try asking on the forum here in the market section. I always recommend hobbyists first because they know the background of their shrimp, and are serious about good husbandry. The members here are typically pretty honest.
A couple hints to help you-
Look at their number of posts. Someone who posts often probably won't do you wrong. They have a reputation to maintain.
Look at feedback. While one may not have many if any negative feedback due to paranoia, if they have lots of positive feedback- they are probably doing something right.
Ask for a picture of their colony. Often only the best shrimp is pictured to sell their shrimp. Perhaps their whole colony doesn't look like that one. Also, some shady sellers will use someone else's picture to sell their shrimp. If you have suspicions, you can be bold and ask for a note with the date to be in the picture of their colony to make sure it is their picture.
You can also just gamble. Sometimes you know a deal will happen if you don't act fast. In that case you can do the quick deal, but be prepared for the shrimp to come in not at all what you expected. In other words, hope for the best, but expect the worst- and you won't be disappointed.