I'd probably disagree with ya there d_m, more so on BL then generally. I reckon there'd be enough old schoolers on here who've been around long enough to see most (if not all common) cutters and neat methamphet (if not the specific enantiomers), and could discern the difference (relatively) easily.
Don't forget not everyone BL use/know/study drug recreationally

A few formally study/work in the field also (and hence have access to things us laymens may not

).
As d_m mentioned, it's not just
one thing which would give you the answer - smell, taste, "type of" smoke produced, melting temp., recrystalisation temp/time, "constituent division/combination" (whether the cutter easily/quickly divides from the active drug - common with cutters such as MSG [which has a low melting temp compared with methamphet] - and whether it recombines), plus a few others. Well, these are the "signs" I use!

However, more knowledgeable people on here may be able to give more in-depth examples/suggestions.