VW's of that vintage burn quite clean when all is working properly and would have no trouble passing. That said, there are tons of common problems that affect cars as they age which would cause them to flunk a smog check test. Electrical issues are common on digifant igntion (bad plug wires, timing set wrong, broken and/or corroded grounds).
If it's not running hot or blowing black (running rich), blue (burning oil), or steamy (radiator coolant leaking into the engine and out the exhaust from a head gasket problem) smoke out the tailpipe it should pass... but there are of course exceptions...