Disregard your fear of not being good enough. You don't need to necessarily be able to play tunes when trying out an instrument.
Once you have seen something that you like the look of, check it over to make sure it is all sound. Nothing falling off etc. Everything where it should be.
Sit down with the instrument, preferably somewhere quietish, you don't even need to plug it in at this stage. Play say the open E and then every single fret all the way up the neck. Repeat for each string. You are checking for fret buzz, dead spots etc. Also listen and check whether it is resonant and sounds good acoustically, if it does. then there is a fair chance it will sound great amplified.
Now plug it in to an amp, set a volume that you are comfortable with and just repeat the above. You don't need to do anything else. You don't need to try to impress the salesman with slap bass antics or double thumbing
If you like it at this stage, perhaps try it on a strap and check for balance/weight.
Then impress the salesman with your bartering and negotiating skills.
Good luck.