Look for traces of redish-pink color stains around the sink drain, shower drain, or in the bottom of the toilet. If you see any traces of red liquid stuff that might have dried up, that would be RV antifreeze, which means your line have been winterized.
You can also turn the on-board water pump on (with fresh water tank empty). If the lines have been winterized with RV antifreeze, there is probably still enough in the lines that the pump will push a little air and you should see some pink fluid, kind of spit or drip from the faucet. If so, it's definitely been winterized.
Basically, look over the faucets, outlets and such and see if there are any traces of the pink-stuff.
About the tanks. All you can do is drain them, or dump them. Once empty, you are pretty much done with the back, grey, and fresh water tanks.
Also, if you see any redish or pink stuff flow from the grey or back tank, that's a really good indicator the lines are winterized.
Check your water heater. Remove the drain plug (some are just a plug, and some are anode rods), either way, remove it and make sure the tank is empty. If it is empty, that's another good indicator it's been winterized.
