It's kinda tricky to describe, but once you know what to look for it's pretty easy. The dorsal fins on the girls are shorter and rounder, while the males have longer, pointed dorsals. On pearls, the males will also have red on their chest and depending how old they are they'll also have extra fringe trailing from their lower fins.
Here's some pics I was able to find...
General female signs (this ones a three-spot morph of some sort):
Pearl gourami (female top, male bottom):
You can see a slightly exaggerated bit of what I mean by the pointy dorsal fin on the male pearl there too. Opalines aren't quite so obvious, but the same basic principle applies. If you mix pearls and opalines, get girls only and at least 5-6 total so the pearl doesn't get bullied too much. Opalines are decidedly more feisty.