For a guesstimate – if you can pick up with one hand it weighs < 5kg
If you need 2 hands to pick it up it weighs<15kg
If you can only lift one side of the artwork on your own it weighs<30kg

A more precise method is to weigh yourself holding the artwork. Stand on your scales and check your weight. Then pick up the artwork and note the weight again.The difference between the two is the weight of your artwork.