That's actually a really good question, and it really depends on the rock. If you're at all familiar with different rock families and can tell what you have on hand, you may be able to at least make an educated guess about whether a rock will polish up or not. For instance, agates are really hard and usually polish up really well regardless of what you do to them. Other rocks like basalt may not polish necessarily, but will get smooth and will hold their shape decently through the process. Also, this probably doesn't hold true every time, but lake/river rocks that have held up to being tumbled for years naturally will likely at least hold up to the tumbling process you put them through, although whether they'll polish up varies from rock to rock.

So yeah, unless you can tell some characteristics of your rocks or actually know what they are, it seems like a crapshoot. This is definitely true for the next batch I have going in...about half are things I've identified and I'm pretty sure will polish/smooth nicely (agate, petrified wood, basalt), and the rest is stuff that I picked up hiking that seems hard enough to stand up to tumbling, but I have no idea what it is and whether it will actually polish.