I didn't watch carefully when mine was installed, and after they left (!) I realized they'd badly damaged my drywall in one spot. It will be covered by backsplash, but it was a big mistake. I used honed absolute black, so I didn't have any seam or pattern issues, but I would think you'd need to be on top of those if you have granite with lots of movement.

You need to make sure the sink is installed per your specifications.
You need to make sure they cut the holes where you want them - for faucet, etc.
You need to look at the seams before they leave and make sure they are flat & almost invisible.
You need to be sure there are no BIG GAPS anywhere...if they templated correctly there shouldn't be.
You need to run your hands over every spot to make sure it's smooth with no cracks or chips.
You need to see if there has been any damage to anything else and note if there has and also point it out to them.
That's all I can think of right now. I'm sure there are more things. I was right there watching and every time they went to the truck or finished one part, I was there looking at what they had just done. But,I stayed out of the way otherwise.

get a level and use it.

A level?

Wouldn't the granite be level if the cabinets are?

First of all, good luck with the granite installation tomorrow. Having gone through one myself, I do know this: make sure the installers use caulk that closely matches the granite or your seams will stick out. Make sure they also put sealer on it after it is installed. And...make sure you inspect everything when it is done and the installers are still there. I agree with all that Suzieca said, as well. A mistake was made with my granite and as a result they had to fill in a hole. I now have a stainless "rub-out" thing that looks like soap next to my faucet.

Oh yeah, and this too...make sure you can open your drawers and any appliances that are under the granite such as the dishwasher, trash compactor...whatever.

A level?

yes.

Wouldn't the granite be level if the cabinets are?

sure. theoretically.

Make sure the dishwasher is secured to the countertop. Look for damage to surrounding cabinets, walls and floor.

We have our microwave cabinet on the end and because there wasn't much structure to the interior of the cabinet, the top actually shifted when they put the granite on as they drug it in place. My DH noticed it right of way and was able to push the cabinet back into it's orginal shape. It's right next to the opening where the stove will slide in and that's why he noticed it.

Sometimes you will have wall damage if the wall is not square as was the case in one corner. No big deal though since the backsplash tile will cover it. You want the granite to fit as closely to the wall as possible because you may not be able to fill in gaps that are too large.

Ensure they have all the overhangs the same depth and that the depth is enough. My sister's installers shorted her and her counters just cover the cabinet tops. My guy was very generous on my overhangs. If your overhang is too short, when you have any spilled liquid on the counter, it will run right into your cabinets, depending on your cabinet style.

If they can, make sure they fabricate the counter outside and not in the kitchen. Mine did 99% of the work outside and only fabricated one edge after the granite was in place.

Shouldn't the dishwasher be secured to a strip right above it and between the neighboring cabinets? I would think you would want the extra support and be able to switch out the dishwasher without worrying about the countertop. Just a simple 1 x 3 filler strip.