I could be wrong, I'm guessing as much as anyone, but the best way to identify what a pitch was intended to be is by looking at it's velocity. Sometimes a pitch can be intended to be one thing, and react like an other. I'm trying to guess what the pitch was intended to be, as in, what Stroman would call the pitch.
For Stroman, according to pretty much every scouting report, there are 5 pitches he can throw:
Fastball- Mid 90s
Cutter- High 80s/Low 90s
Slider- Mid/High 80s
changeup- Mid 80s
Power curve- low 80s
The bolded pitches are easily identifiable, Stroman even said it was a curve that was doing the damage, and when you watch the tape you see the nasty pitch we are all raving about was consistently in the low 80s, so it was a curve.
The Changeup/Slider/Cutter can be confusing though, since all three sit in the same general range of velocity (and excluding the slider down and out of the zone which he didn't throw much on saturday since his curve was so effective) they all generally have similar type movement and stay in the zone (cutter, change, and slider for a strike).
The main reason I am guessing the high 80s pitch for a strike was a slider is that I know historically Stromans slider is his second most used pitch after his fastball, and I know one of the strengths of it is that he is capable of throwing it for strikes. It seems unlikely to me that Stroman wouldn't throw a single slider, so I'm guessing the pitch that read electronically as a changeup (the pitch he historically uses the least of the 5), was in fact his slider being thrown for a strike. And when you watch the tape, that's what it looks like, it bends and moves alot more than a common changeup (look at that horizontal break).
So looking back at his Saturday performance, it appears to be Stroman was generally Fastball/Curveball, with some Slider/Cutters mixed in for variety.