Actually, drug tests can't differentiate between Amphetamine medications like Adderall & Meth. Years ago I had a really bad Meth problem & just so happened to be being prescribed Adderall the entire time as well, so I've had tons of doctors & tons of drug tests I "passed" confirm the fact that drug tests do not differentiate between Meth & Amphetamines.
However, having an Adderall 'script still isn't necessarily a "free pass" to do all the Meth you want & still pass drug tests. Since drug tests obviously report the exact amount of the substance that's in your system, & it's not incredibly difficult to tell a normal, "therapeutic" amount of Amphetamines that's in your system from an incredibly high amount of Amphetamines that's consistent with Meth abuse. Still, though, unless you're the biggest tweaker on the planet perhaps, all that would lead to is suspicions, since it can't be proven as a fact whether somebody with an Adderall 'script is using Meth or not based solely on the level of Amphetamines present in their system.