I don’t work with spark, but to me spark is sort of an inbetween if you will, between a specter and a yellow belly. They have some of the flaming yb’s have, the same head stamp as a yb, but their pattern tries to emulate specter, so i get the confusion. That confusion is why i have stayed away from sparks, and asphalt/gravels too. I can almost always ID specter and specter combos from yb and yb combos, because they are quite different. Specters main marker is their square and rectangular aliens that extend clear to the belly. In general specters have very few flames compared to yb.
Specters tend to be striped with spots inside some of the striping, some sparks also have this feature, so no help differentiating the two there. Same head stamp too, all of the yb complex head stamps are very similar, so no help there.
Having not worked with spark, i really can’t say how to 100% differentiate spark from specter, but in general, spark with try to emulate yb more than specter. Most sparks and most yb’s have a lot more flaming than specters have.