Which wire is the ground in this light box?
Here's my best guess at the wiring based on the photo. Looks like you've got one hot/neutral coming into the box from the panel (or another switch/outlet), and you've got 2 hot/neutrals going out of the box to other switches/outlets.  Since these are all joined together, it doesn't really matter which one is coming in and which two are going out.

Then it looks you you have a white wire connected to the bundle of hot wires, and that white is going out of the box to the switch, and then a black wire returning from the switch, which goes to your lamp. This is pretty common, but that white wire should probably be marked at both ends to indicate that it's actually a hot.

Based on what I can see in the picture, you do not have a ground wire. It's possible that the box itself is grounded, either with a ground wire screwed to it that's  not visible in the photo or by metal conduit, but based on the yellowed insulation of the white wires, my guess is that your house is old enough that no ground was required at the time, so there is not ground wire.



Update based on your close-up picture, there are ground wires, they are sort of terminated to the box on the strain relief clamps. If you have enough slack, it would be good to wirenut them all together securely with one pigtail that goes to the box and one to your lamp. Otherwise, make sure they are all securely fastened to the box, only one of them (the one that leads back to the panel) is a "real" ground, the others are providing ground for downstream lights/outlets, so  make sure that all all them are securely fastened.