Always look for a recomendation/ testimonial. Ask can you get in touch with past clients? Look for owner & dog classes
BeauZak is right, look for a safe & secure training place, insurance etc.
The rest is up to you, what to you want your dog to gain from lessons.
Do you want group/private lessons?
Do you want weekly sessions over a number of weeks or do you think a once off will suffice?
At the end of the day , it's owners choice & with more trainers out there you can make a well informed choice.
Most impotant is to think about what you want to get from the lessons & ask relevant Questions. Any trainer with their salt will answer honestly.
By the way, it's definitly not a quick fix to making money, so anyone thinking it's recession proof is far off the mark.