Contact the supplier or dealer for that brand - have make model, s/n, mast type, attachment, tire type & configuartion (sin/dual, solid pneumatic) etc & fork length & section size ready. Cascade has a "capacity" guesstimator on their website but it is just than a SWAG and they will defer you to the lift truck manufactuer or their representative for actual ratings.

There is a way to use the inch-pound rating to guess at the capacity (used to do that back in the 1960's then OSHA came along in the US) but it does not take into consideration - mechanical limits (like fork section sizes and other things) and ratings to full lift height.