Yes the difference is small in the upper levels, it can be hard to tell, and the bot name only a guide as they can be any speed trainer.

The first day as soon as server is up is good time to look, as Burle said above. The best ones seem to have the shorter auction times. And also he's right, look for ones that their value is as high or higher than your best current players.

So, beginning of season or early in season day, bot team, short auction time, highest price will usually be a good trainer. I have done ok with it...
I have also bought a 20 y.o. gen that spent time on another team first, the value was so much I took the chance and it worked.