You may be 'no chemist', but you give good advice IMO! Finding interesting pills and powders in bags is rather fascinating.... and tempting.... and exciting even. I have in the past been to some pretty wild raves, where all sorts of things were found by me and my friends. Being rather reckless by nature, if it was a few nondescript pills, I had no hesitatin in trying half a one (to start) and chewing it very carefully to get the taste. I thought, as logically as possible when partying, that whatever they were, nobody would make any sort of pill (legally or illegally) which was going to kill or seriously damage anyone on its own... well, not unless you were incredibly unlucky to find some WW2 SS Veterans were at the party, and had mislaid their cyanide pills. In other words, the risk was negligible (80+ year old Nazis seem to avoid dance music, lightshows and luvved up crowds IME?), and though most such 'finds' were pathetically weak MDMA mixed with ephedrine or poor quality whizz, a few provided rather nice, surprising bonuses!
That was however at good natured, luvved up parties back in the 80s/90/s, and things have changed. Even then, any bags of powder were another matter.... usually the sort of speed only any use for sweetening coffee. Almost always binned! I did investigatate even so, and had I not, would have missed a generous helping of excellent cocaine - I'd guess a £100+ deal, dropped by some poor sod who was no doubt gutted (and may have gutted me had they known). Needles to say, I have also lost things myself, like the odd precious half E 'escaping' into an impossibly dark corner whilst being bisected, and a couple of small bags of MDMA pills which somehow vanished from my drug-crazed fingers on the dance floor... somewhere? Although disappointed and grief stricken by this, I always hoped that someone would at least enjoy them, just as I would if I found them...
Back to that powder of yours, I would not touch it unless you are 100% certain it is meth. Even if it is, judging from where and how you found it, I'd suspect it is fairly poor quality, and won't provide you with a wonderful experience even if pure. As for 'getting addicted' after trying it once, don't worry, you won't, and unless feeling suicidally depressed, can't. I am a very long term amphetamine user, it was prescribed to me for 14 odd years, being the only thing that allows me to have a bearable life, by overcoming the catastrophic tiredness which my refractory, 'atypical' depression causes. Unfortunately that miserable, inconvenient and grudgingly given 'privelege' was withdrawn a few years ago, and I was 'treated' (ins. bitter laugh!) at an NHS 'addiction centre' for my 'addiction', by slowly withdrawing the dose over a year, and leaving me to live my last years in misery, exhaustion and suicidal despair. No treatment for that, no help offered - just as well I'm too weary, broke and anergic to burn the place down with the staff inside, which would be doing the world a favour.
Despite that, and the fact that I have taken well over four times my own bodyweight in pure Dexedrine, with no ill effects, mental or physical (to the annoyance, fury and astonishment of the medical profession!), I refuse to kill myself for the convenience of a bunch of lazy quacks, and have managed to find a cheaper, fairly reliable source for my essential medication - with the benefit of not having the police and Home Office breathing down my neck, and no tiresome, pointless lectures from stupic medicos.
And so back to your query (having had a good whinge!). I prefer amphetamine, it feels more solid, predictable and psychologically 'suited' to me, but of course I am not a typical speed freak - there is no pleasure or joy in it for me, only relief from my crushing depression and tiredness. In other words, there is no 'high' (even at 200 -300mG of dexedrine, probably the equivalent of three + grams of 'street' racemic sulphate), merely the absence of 'low'.
Quite recently I have been taking methamphetamine instead, as a substitute, and the difference is pretty noticeable to me. Not easy to describe, but it seems (gram for gram) a little stronger, yet somehow less psychologically effective. I took a dose this AM, and it seems to take longer to work, and is less 'inspirational'. In other words, I sense a fairly pleasant, but sadly unproductive 'dreaminess' in a way, and the 'get up and get going' feeling of dexedrine is badly muted. It keeps me awake and lifts my mood, but the wakefulness seems to go on too long, when I am mentally too tired to do anything useful. A distinct lack of concentration, purpose and resolution, and all the projects I had hoped to get finished (or started) just sit around gathering dust. A lack of enthusiasm and 'mental stimulation', almost as if I had smoked a big, fat turbo-skunkweed spliff for breakfast - which I never do! Despite all those vague, introspective criticisms however, it hasn't stopped me writing reams of crap as you can see... so something's working?
All in all, I'd give it a miss I think. If you are unbearably curious, I'd suggest you obtain some from a reliable source... though I'd not bother myself, I only take it since there's no option! As a final grim note, that little bag might possibly contain PCP, Ketamine or GHB. Or Mephedrone.... and if it does, any of those, addiction will be your last problem! Good luck... I think you know what to do...?