I always get a kick out of how some people have to talk about an expensive item they own to somehow justify a silly position. Sounds like having to make up for something. lol
To answer your questions-
You know they are a pro the same way you know any other pro is legit. You ask for a reference, get a referral, check their state license, and ask if they are insured, etc. It takes about 2 minutes on the phone to verify this. Then you ask them if they provide a guarantee of their work. Same as hiring any contractor.
Then they come out and review your lawn and weeds in about 10 minutes, they create a plan, they explain the plan and your options, and then they do the work agreed upon. If the weeds aren't dead in two weeks, call them back out to make good on their guarantee. I'm sure Cleveland has 250+ licensed applicators.
I am not opposed to a person educating themselves, but sometimes the cheapest education is to start with a pro. There are at least 20 different variables that need to be considered when killing weeds to get the desired result with the least negative effect on your grass and the environment. You could probably get your lawn sprayed for $100 with a guarantee or you can spend 5 times that amount and screw things up so bad that it would cost 1,000's to repair. I see it happen a dozen times each season. 10 years ago I would feel bad for these people but now I just laugh. penny wise and dollar foolish