Checking 'plumb' when mounting bike on trainer
There are several options, ranging from low tech kludges to actually solving the problem. So, starting from low end:


Loop a string around your top tube and tie so that loop is loose and the string can move. Tie the weight to the other end of the string. Now you have a plumb that hangs at the center of the tube.
Get a spirit level. If the solution needs to be free and you have a smart phone, there are also apps for it.
Higher tech variation: Get a laser spirit level and fix it to the frame so that the line shows at down tube. Now you can check the bike position by just looking down.
Actually solving the problem, option 1: Forget the trainer and use rollers instead. You will now maintain the bike upright automatically.
Actually solving the problem, option 2: It sounds like you thorougly dislike cycling. Try some other kind of exercise.