Drug tests vary in sizes and shapes. Some just test for broad categories such as "opiates", 'marij", "coke" "amphetamines" and etc.
However, most tests today have gotten pretty sophisticated and don't cost much. They will break down the results by broad category and then sub category....I.e "opiates" and then from there hydro, oxy, fentanyl, & etc. The results show a "level" that's similar to what you see as dosages.
Muscle relaxers, most often "soma" do not show up on routine drug screening, but don't take my word as gospel. First and foremost, drug screening is looking for controlled substances.
Also, it's important to note that with drug screening, there is a "threshold" that must be crossed in order for you to test positive. Meaning that the test is designed to measure substances at or above a certain level. The logic is that it's better to let a few positives slip through undetected than falsely accuse someone who hasn't taken anything. For example, poppy seeds in bread can show up in a screening as pot at very, very low levels. So, they set the threshold much higher than that, so there is NO WAY someone who eats poppy bread will test positive for pot. That doesn't mean that you can go out an abuse drugs and not get caught, but rather if you have very small levels in your urine, or have taken something long ago, it will be more than likely undetected.
However, I must warn you that I wouldn't fool around with these things. These tests are fairly sophisticated and designed to catch people who slip up. Use your favorite search engine to search "how long xyz shows up in urine drug test" and you'll find all the info you want, including elimination times.
Finally, I highly discourage anyone from trying to "beat" a drug test. Not only do testers today either watch you pee, or send you into a secluded room, but the screenings test for all sorts of "indicators" or things that are normal in your body....creatine, enzyme levels and etc. Anything out of whack can indicate that you've tampered with the urine or added some masking agent. Synthetic urine doesn't work either.
Many drug screening tests today are making sure drugs are IN your system....i.e pain mgt clinics. If you're scripted something and it doesn't show up in your system, they may dismiss you.
Good luck and I hope I have provided you with some basic info.