I bought fennel last weekend and I haven't cooked it yet. Is it still good?
You can tell from the color: has it started to turn brown. You can remove the outer leaves and the inside ones may still be fine. Also you can tell by the firmness: if it seems flexible it probably won't taste as good. You aren't in any danger from eating. It's just a question of optimal taste
If it was stored well, I'd say it's fine. Just look for obvious signs of spoilage -- mold and mushiness come to mind -- before you use it.
Unfortunately what you are most likely to lose will not be the bulb but those fluffy, fern like tops (which hopefully the grocer didn't hack off). There is so much flavor in the green stuff that it's quite sad to lose it.