Thanks for the replies, everyone. I went out just now and looked at it again after it had cooled down completely. I fear the worst.
No coolant is visible in the radiator, but the overflow tank remains nearly full, even after fully cooling down.
I blew through the hose connecting the overflow tank to the radiator - seemed a bit tough to blow through at first, but it's fine now. I sniffed the radiator fill neck with the cap off, and it smells like exhaust. As far as I know, the only thing that can make your coolant smell like exhaust is either a bad HG or cracked head / block. So, I think it's done.
I guess now I'll have to either read up on how to change a head gasket, or bend over and pay someone to do it / help me do it that isn't too far away. At least it got me through winter, I guess.