Transparent colored or clear glass with opaque white swirls or ribbons.
Although it is tough to positively ID every slag/onyx marble, here are a few things to look for. However, since we have learned different companies can share the same characteristics, none of this is "cut in stone".
Peltier: swirls of thinner white glass, also known as "feathering".
CAC: one or two seams and the colored glass seems to be a little brighter.
Akro: no real defining characteristics, but if it doesn't fit into any other category, then it is likely.
MFC: the "9", tail, and the cut line. But no red ones.
Hope this helps.
thanks.