the_dude
If it's really cold outside it will get a frost line right at the level of the gas. Outside of that you'll need to see if the propane company can install a gauge.

I AM
If the tank has a gauge, it should be under the metal lid on the top of the tank where the gas line comes out. Thats where you can check. If one isn't there, have a propane co put one in so you don't have to guess. Have them check the pressure comming into the house while there at it.

Dustoff
Feel the side starting at the top. When the tank suddenly feels colder, that's the fluid level. If your hand is not good enough use a spot thermometer, some are like a laser.

Trish
There should be a hinged cover over the guage on the top of the propane tank. Lift the cover up. Look at the guage that reads: 10, 20, 30, 40, 50, etc. Look at where the needle is pointing. If for example, it is pointing to the 50, take that number times 10. If this were the case, you would have 500 gallons of propane left in the tank. If this were a 1,000 gallon tank, your tank would be half full.

KirksWorld
I've never seen a 100 gallon or larger tank that didn't have a gauge under the lid. It is usually in percent. You'll want to call for refill somewhere between 25 and 15 percent depending on how cold it is where you are at.

etcher1
Here's a link that will tell you how to read a propane gage. Other links can be found if you google propane tanks. Hope it helps.

tony a
pour very hot water over it .propane boils at -22 degrees it will leave a frost line .you do the same with propane grills so that u know weather to make hot dogs or baked chic kin!

opie with an attitude
If there is not a gage mounted on the tank (usually about the size of a quarter) then I would have the tank replaced entirely, because it is probably too old to be safe. Gages have been required by law on any tanks since the 70's. If there is no Gage on that tank then you have more problems than not knowing how much gas you have but a potentially explosive problem.