I have about 3/4 gallon of fuel that's been stored in the tin container it came in in a cabinet in my garage (completely out of sunlight) for about a year and a half. I'm in southern California so the temperature is a pretty constant 50-70 in my garage year round. I plan to get back into nitro soon, and I'm wondering if you think there's a good chance of the fuel being bad, and if I do run it will it cause engine damage or just not run properly?

Junk it get fresh fuel

Throw it away . Old fuel can separate and leave you running no oil for lubrication.

For what fuel costs it's not worth the gamble. It's pretty dry there but the biggest concern IMO would be moisture content causing temps to spike.

There was an artical a few years back where the guy that owns Byrons Racing Fuel dispelled a few myths about nitro fuel .

In it he says that as long as it was kept in an air tight container it should be good for quite a long time , the oil does not separate from the fuel , nor does it lose any power , it can get water in it because its mostly alcohol and that is Hygroscopic and attracts water , but if it was capped then it should be OK .

That's the biggest concern is water absorption. You are correct about the hygroscopic trait. I've noticed that most fuels that have absorbed water turn a little yellow. So blue thunder starts to look green, sidewinder yellow/orange, etc.

As far as oil separation I always shake my fuel before removing some from the jug. The oil won't go anywhere; it's still in the jug.