Hi Martine,
Normally, it's easy to determine whether a bead was cut off from a drawn cane. I'd say the vast majority have concentric layers, even if there are only two. Solid color drawn beads also were made - but most of those were much smaller, typically known as seed beads. Smaller drawn beads were normally re-heated on big trays to round the edges.

Millies and "fancy" beads were made one by one in a torch. The shapes can be round, oval, cylindrical, etc and the ends can be rounded during the making of the bead. For most but not all of these you can see the swirling of the glass which was wound around a mandrel.

So, the two processes are very different - in one case, many identical beads are cut from a cane, and in the other case, individual beads (which are not identical, but can be close) were made one at a time by a lampwork artist.

And remember, a speo means that the bead was placed on a rod for re-heating. For reasons of labor intensity, this was only done for larger, more valuable beads. Just because smaller drawn beads were re-heated does not make them "a speo" beads. So the oval melon beads you mentioned were likely not re-heated on a rod. That's why the holes often appear squished out of round and irregular.

Hope this helps.