Chances are if your 80 hasn't ever had one, it now needs one purely based on age. Every B3 owner has to do this its almost a rite of passage!!
Yes, its a dash out job. The first time you do it it will take more than a few hours and you will find other bits and pieces to do along the way, heater flaps, wiring abortions and such like. There have been some very good how-to guides written over the years.
You'll know its gone if you are constantly topping up with water, wet footwells, excessive condensation, greasy film on windscreen etc. Foam from the vents - prob a safe bet your flaps at the very least have had it I'd say
Find where the lower vents are for the footwell on the passenger side, have a quick feel inside, if its wet there you have confirmed it without taking anything apart. This is where the matrix lives and is its lowest point.
Good luck!