Never use a propane tank where the protective shield around the valve has been bent up enough or broken in such a way that it no longer protects the valve from getting hit. When a tank is seriously rusting, change the tank. It small scratches have begun to rust, repaint with a rust proofing metal paint right away.
Make sure your propane tank has a triangular shaped handle. For details click here.
Telling how much propane remains in a tank can be a problem. You can buy heat sensitive strips where you glue them to the side of the tank and pour boiling water over them. Where there is no propane, they remain hot and where there is propane, they get cool quickly, showing the difference with a difference in colour on the strip.
A more accurate and easier way is to weight the tank. If you weigh an empty tank, then a full one, you know just how much the propane inside will weigh. When you weigh a partially full one, the fill level will be proportional to the weight. In fact you can buy a tank support that helps to hold the tank in place under the BBQ but has its own scale on the bottom. As you can see in the photo above, it has a dial just like the gas gauge on a car that lets you know just how many steaks you can cook before running out of gas.