For a pre OBD-II vehicle such as this one, the best thing to do is get it running as smoothly as possible, using normal maintenance and repair procedures. This means no misfiring, smooth idle, smooth acceleration, not overheating, etc. The same stuff you'd expect to work well in every day driving. There should also be no check engine light. This vehicle will have some sort of basic self-checking, it just won't follow OBD-II standards.
If all of this is working well, then there is a very good chance you'll pass emissions. However, there are some conditions that won't show up in daily driving that could still fail you, and those have to be diagnosed on an ad hoc basis, after the test.
Mosts emissions tests will check NOx, CO and HC. A very basic rundown:
- NOx - Oxides of nitrogen. This means the engine is running too hot (2500F +). This can be caused by poor cooling or fuel mixture is too lean. Modern vehicles also have an EGR system, which exists exclusively to reduce NOx. If the engine is running in the correct tempuratures, and NOx is high and everything else is normal, the EGR system is strongly suspect.
- CO - Carbon monoxide. CO exists when there is an incomplete combustion of the air/fuel mixture. This can be caused by restricted airflow (dirty air filter), or a sensor that is reporting the incorrect airflow (MAF/MAP, TPS), or a faulty O2 sensor.
- HC - Hydrocarbons. This is raw fuel, meaning the fuel didn't burn completely during combustion. This can be an ignition problem such as misfiring, or poor timing. Usually this will show up as a driveability problem.
There is a lot more to diagnosting emissions, as there are a lot of complex systems all working together, and all of the readings have to be considered together, not in isolation. For example, if all three readings are high, and the engine checks out and all underhood systems check out, the catalytic converters may be faulty.