Hopefully this post will help not only you, but anyone else with this question:
If this is for work, either pre employment or random, they will not be looking for steroids. The will be looking for the usual suspects, thc, cocaine, amphetamines, opiates, benzos, and pcp. This is basically because a test to include steroids is more expensive, since there are more QC standards for the testing company to keep updated and calibrated for on their equipment. Since steroids aren't the main cuplrit in loss time, they aren't typically looked for.
If this is a sports related test, you can bet your ass that they will be looking for steroid metabolites as well as the other drugs.
As far as masking, if this is a GC/MS test, you can't mask anything as this will detect anything added to the urine and will raise a flag, if not fail you for adultering. The cleanout drink sold in head shops are bunk bucause they instruct you to take the product with a ton of water. Water alone can dilute your urine to undetectable limits of the test.
The drawback to diluting is that they probably will do a creatinine count, and if it below the standard range, you may fail for dilution.
If this is a kit test, they will just add a few drops of urine to the kit and will show a line by each metabolite listed if positive. This test can be diluted without much fear, however they will check the temp of the urine, and it must be between 94-97 degrees F. This is standard for all urinalyses to make sure it came from the individual and not the tap or other place.
There is another test kit which is just a cup with the list of metabs on it and reads the urine temp and indicates with a dot or line the positives. Much like the dropper kit, but there is no transferring of the urine, which is popular for a quick random in some companies.
However, if this is a test done for work in a saftey sensitive environment, you can be sure that it will be on a GC/MS. This is one of the most effective way to detect, quantify, and rule out false positives. It is very precise, and hard to beat. You're either clean or you're not. This is also the case if they are testing for sports.