Another good way to tell is look at the intercooler.
If the BOV is bolted to the intercooler it's a '99 if the intercooler has a elbow and a rubber hose that runs to the passenger side and down, the BOV is just bellow the 'cooler, it's a '98.

Big difference's under the bonnet would be cylinder heads, manifold and ecu/engine harness.
These parts wont interchange between the two models with out modification.