Step 1: Don't freak out. Your liver is the most resilient organ in your body. Its job is to take impurities and filter them out; in our evolution as a species, chances are it was getting a lot more nasties than a bit of alcohol and meth (or whatever kind of damage you are doing to it). So long as your eyes aren't yellow and you aren't noticing any other textbook signs of liver damage, you are probably fine.
Step 2: If you are still worried about the health of your liver (and it's good to have a healthy amount of concern), drink a lot of water, cut back on the substance intake, take multivitamins, eat vegetables/fruits, and go ahead and get some of that milk thistle (I've never heard of it, but if so many people here think it's good, I trust them).
Step 3: If after all of that, you are still afraid of damage, go to your general practitioner. Get some bloodwork done. It is a really easy blood draw that can determine how well your liver is functioning. If there is concern of damage, they will do an ultrasound. One time I went to my doctor after a binge, and she took some blood, and later informed me that I had a high level of xxx enzyme (whatever it's called). She wanted to do an ultrasound, so I agreed, and went clean for a few days (3 max). When I went to the ultrasound, all was fine, no damage noted.
If you're below 40 and don't have any of the signs of liver damage, I would wager large amounts of money that you are fine. Don't freak out, and talk to a doctor if you do freak out.