There are walk-in labs in most cities where you can have blood work done without a prescription. Only downside is that your insurance company likely won't pay for it. Depending on your insurance, paying for the test yourself may be cheaper than paying for a visit to the doctor to get a prescription.
Do a google search for walk-in labs in your area if you don't want to go through a physician. If you're not taking testosterone and you want to check your natural levels, get an 8am total testosterone level. If you're taking testosterone and want to know your levels, the time of the day that you have blood drawn doesn't matter.