It is correct that the knee has to be covered in any position – standing, sitting with/without knees crossed, walking, going up stairs, etc. Best bet (other than trying the skirt on and doing all those things and having your mother/sister/a friend watch to make sure your knees are covered): Measure the distance between the bottom of your knee and your ankle. Make sure the skirt hits halfway between and you’ll be fine.
If you want to buy skirts without trying them on in the store, so measure from your waistline to that half-point, write down the measurement and then take a tape measure to the store – if the skirt matches that measure from the waistline down, it should be fine. Hatzlachah!