Usually they have a list/paper, with spec info pasted on the inside of one of the cupboard doors. I don't know if it will state the tongue weight, but it should have dry weight and maybe even loaded weight. You should be able to figure something close from that.
Maybe somebody else will chime in here with a little better answer for you.
If you can find some scales around you, nothing beats actually weighing the camper. You can guess, you can suppose but a you can easily be a few hundred pounds off.