Can laboratory drug test tell the difference between methadone and heroin?

The answer to this question is YES!

Depending on the type of test given, it can tell the difference between pretty much ANY two substances.

They will never tell you how in-depth the test will be, for obvious reasons.

If you are subject to a drug test and you are using something other then what you have been prescribed, it will more then likely show up.

I'm not saying that tests are always 100% as there are always exceptions.

Heroin -----> C21 H23 NO5

Morphine --> C17 H19 NO3