You can path out his route in your mind based on your knowledge of his jungle champion (clear speed, sustain, etc)

From this information, you'll know roughly where he is on the map for the first few minutes of the game.

And from this information, you'll know when it's safe to play aggressive in lane and when you need to play passive.

Then, the next time you see him, you get new information like how many creeps he killed, his health level and items, and the time between when you first see him and when you're seeing him now.

From this info, you can pull out his exact route, whether or not he shopped on this route, how many wards he has (important), etc.

From this info, you can roughly figure out when his jungle camps will be coming up, whether or not he took scuttle crabs

From this info, you can figure out whether he'll farm his jungle, or gank or invade (i.e. If he's top side, and you figure out he just took his top camps... he'll either gank top or mid or invade or recall... or go bot side...)

And from this, you can now predict (again) when and where he'll be over the next couple minutes, i.e. From the previous example, if he doesn't gank top or mid (and your team's wards didn't spot him invading [ warding is important ]), you've determined he either recalled or went to clear bot side. You can narrow down these 2 options into 1 option based on things like his current health, his items, and which champion he is.

etc etc etc, The options become more and more numerous as the game goes on making it really hard to track, but in those first few minutes after doing his first camp, there's only so much he can do.

It really is a TON of information you can pull from simply seeing the jungler's starting camp... but it's also difficult to track it all accurately