Taking your last question first: Call around and talk to vets in your area and try to find one who has education and training and experience with herps. Lots of vet receptionists will say the vat has experience and so it is fine to ask for references, what types of herps, how long, etc. Than you can ask how much a general check up and sexing would cost.

Because your snake is an adopted, new to you snake, I think it is wise to get him to the vet for a once over. The vet can open his mouth (even you can) and look for mucous and/or bubbles. Without being able to see and/or hear your snake, it would be difficult to diagnose early stage RI.

So, start calling around to area vets!