The fast trainers are usually popping up in the mornings as we said, usually 1hour after server restart(new day) and Usually fast trainers belong to teals w/o a shirt sponsor & weird names begining with F.C or ending with A.C and so on.. there are many ways to spot them.
Once you buy a fast trainer you'll get a message, when you try to check the team that sold that player, you won't be able to see it..saying
"Sorry, information for this club is not available"
Click image for larger version.  Name: ex.png  Views: 30  Size: 72.8 KB  ID: 33779