How much will it cost to get a transmission checked?

It depends on the car, drivetrain type (front or rear wheel, 4x4's), and transmission type. Taking apart an automatic transmission is a substantially more difficult task than taking apart a manual. Other than that, a good and decent mechanic or knowledgable friend can tell you what is wrong most of the time by driving it. Usually msot transmission problems present themselves when your car drives differently. Does it slip? Shift easy or hard? To simply remove a transmission is labor intensive, around two to three hours of labor depending on the drivetrain. Regardless of transmission type, the difficulty of removal is rear wheel drive, front wheel drive, then 4x4, from least to greatest. Your best bet is to take your car to a good shop and I personally recommend one that specializes in transmissions. A diagnostic shouldn't cost more than 1 hour of labor in my opinion, but hopefully you can get an answer with a diagnostic being performed and your transmission not have to be taken out.