Assuming he's a qualified mechanic, he's probably going through the motions of checking each line one by one. However, if you want to do it yourself, you'll need a decent ammeter - capable of handling at least 10 amps. Start by charging your battery - disconnect the negative cable, and with the car off, connect the ammeter between the negative terminal on the ammeter to a good solid ground. It should read less than one amp while the car is off. If it reads a lot of current, you have something drawing current while the car is off,. Start by disconnecting fuses one by one and watch the ammeter. When you see a significant drop in current, you'll know which circuit is causing the drain, then you'll know where to look in more detail.

Most people have these sorts of problems when they install stereo equipment and hook it up wrong.

Or, it could be a bad battery, not capable of holding charge....double check that.