I'm a mind your own business kind of guy. The best route is to tell the customer that he is not liscensed and that since he applied fert that he messed up your application schedule on his lawn. If the customer wants to use an unliscensed person it is by there own choice.
If I spent time worring about whether someone was legal or not I'd probably go out of business. Best thing to do is to inform your customers that you are a legal applicator and hopefully if they bid you out they will remember and ask the other companies about there liscenses.
Good luck. Be careful about buring bridges you might get stuck!