Poke test: lightly flour a finger, press half an inch into the dough. If the indent springs back fully, it needs more time. If it stays, you're overproofed. If it slowly returns halfway, you're there.
Volume test: dough should have grown 50–75% in the bulk ferment. Not doubled — that's usually too much for sourdough.
Jiggle test: shake the bowl gently. Properly bulked dough jiggles like set custard. Underproofed dough is stiff; overproofed is sloppy.
Use all three together. They correct for each other, and after a dozen loaves you won't need any of them consciously — you'll just know.