Had a little trouble with this, USA being so adamantly non-metric, but this is what I figured out:

If you can, just fill the pot or an equivalent with water and measure.

If you can't, 1 liter is 1000 cubic centimeters (ccs), so figure the area of the bottom in centimeters, 3.14 x the radius squared; then multiply by the height in centimeters, this give cubic centimeters, then divide by 1000 and you will have liters.

There are probably some standard sives , so once you have figured this out, you will be able to tell pretty closely for other pots.