That's going to be a tough one, there are a number of different chemicals in the neonicitinoid family: acetamiprid, clothianidin, imidacloprid, nitenpyram, nithiazine, thiacloprid and thiamethoxam, etc. Each one, in turn, is sold under a variety of different trade names. Imidacloprid, for example is sold under at least 20 names (Admire, Gaucho, Intercept, Mallet, Maxforce Quantum, Merit, Nuprid, Premise, Winner, etc.). Unless the nursery grows its own plants, chances are the employees won't be able to tell you what pesticides, if any, have been used. It's not the sort of information wholesale plant producers are inclined to just volunteer.
One last thing, the presence of bees on plants tells you nothing about whether the plants have been treated with a neonicitinoid or not. For example, I grow a lot of Baptisias in my garden. Bumblebees absolutely love Baptisias and I've been told by a bee researcher that I probably see every bumble bee living within a 2 - 3 mile radius of my house during the Baptisia bloom season. Let's say I decided to treat my Baptisias with Merit (I don't) every year to control some theoretical pest. It would have devastating consequences on the local bumblebee population. But, I would likely continue to see "lots" of bumblebees during the Baptisia bloom for several years until the population finally collapsed completely. In such a scenario, it would be very easy to convince myself that applying Merit to my Baptisias had nothing what so ever to do with the sudden disappearance of the bumblebees.
Seeing bees on plants really only tells you two things, the surrounding landscape has enough suitable habitat to support at least some bees, and bees like those particular plants. In other words, the fact that I have 100's of bumblebees buzzing around my yard during the Baptisia bloom really doesn't tell me much of anything about whether my gardening practices benefit or harm the local bee population.
