okay, first a word from mister sensible: DOOOOONT DOOOO IT. seriously, it's probably the most addictive thing there is, certainly the most addictive thing i evertried and still my absolute favorite.
also important: do you have any opiate tolerance?
sure you can speedball without needles. is your smack powder and not black tar?
okay, if you're determined to do this, FIRST, cut a very small line of the skag. if you have no tolerance, i'm talking mickey mouse tiny. then snort halt of it and wait half an hour. if you feel slightly buzzed, like say after 10-20mg of hydrocodone, you'll have scored some decent gear. if you notice nothing - this happens often - then you probably have standard stuff.
finish off the line and wait a bit longer. if the 'low dose of painkillers' feeling sets in, at least you weren't ripped off. but even if you feel nothing, don't panic or go start shooting drug dealers (i know we all wanted to at some point). then, when you've more or less ascertained the quality of the skag, the fun can begin.
you can also OD on coke, although it is not half as dangerous as strong opiates.
mix two small lines of coke and h. if the dosis was right it'll probably feel better than any other drug you ever took. start low, you can always put more on top. when you can gauge the potency of the drugs, have a blast.
remember: these drugs are both very addictive, and i'd say combining them easily surpasses the addiction potential of one alone.
oh yeah. coke fades faster than h usually, so you can feel fine for an hour or two until the coke fades and only the h goes strong, this has led to OD.
also, this might ruin your life. like i said, i know of nothing more addictive. and this should be self-explanatory, but tailor the amount you consume directly to how strong the stuff is. and it's always better to snort a little more coke than h, it's comparatively less dangerous.