Price is a good indicator, SA is one as well, as most dont bother training players to learn SA's. Indicators they are, nothing more, expensive players can be slow trainers and players without SA can be fast trainers and vice versa. What seals the deal is the club name you're buying him from. If they have names like AC Muddy, Silly, Oily, etc. they'll be fast trainers, generated by Nordeus. (at server reset I think)

I guess it's the only sure way of knowing you're buying a fast trainer from the auction market. The only ones I can afford are the high level 4 stars. For the price you pay for them, you could also buy 3 more experienced players 5-10 levels higher then the fast trainers.