How can I tell if my acorn squash is ripe before cutting it open?
First, as a gardener who's grown these, I can tell you that acorn squash — like all winter squash —  need to be left on the vine to ripen. After picking, they will get a little sweeter as they sit for a couple weeks, but if picked too early, they won't actually continue to ripen.

That said, there's no great trick to picking a good one. If you see some some orange color in the skin, it's almost certainly ripe, though they can still be mature when all green, too.

Ideally, you should be able to trust that the farmer harvested these properly, but if you're getting unripened winter squash, someone probably screwed up out in the field. If you think the store gives a damn, it's probably worth mentioning that you're getting some bad ones.