I started working out three months ago. I am going to the gym 1 1/2 to 2 hours a day, at least 4 times a week. I am not taking anything but regular diet and water.
How do I know when I am working out too much?

yes. You can work out too much.. there are some athletes that train for hours a day... but you have to read your body.. are you drained and tired? Are you getting sick a lot? When you exercise, you put stressors on your body that will lower your resistance to micro-organisms.

Here is an article that discusses the issue.

first of all. congratulations !

are you looking forward to your workouts ?
if you are burned out physically or emotionally, you may start to dread going to the gym instead of looking forward to it.

that might be your first 'hint' that you are doing too much.

Some of the best schools of thought in this area say that you should only workout about 4 days a week but no more than an hour a day. More than that and you are likely doing too much. Of course some of that depends on the exercises you are doing so it can vary but certainly no more than 1.5 hours. I lift weights for an hour 4 days a week and do a different muscle group every session to ensure I don't over train. Overtraining tears down your muscles and doesn't give them time to recover fully and is bad in the long run.