Outside of CA, and as long as there's no equipment inspection, "generally" 96+ models will be tested via the OBD-II port. You can easily check that with a scanner available virtually anywhere. 95- models may be placed on rollers and have a sniffer placed in the exhaust. Different years have different maximum allowables for whatever gasses they are testing for, usually carbon monoxide, oxides of nitrogen and hydrocarbons. Common failure points on older vehicles (I've found, with Monteros anyway) are catalytic converters and, if equipped, the EGR system, particularly when you get back to Gen 1's. Vacuum leaks will kill you, too. Typically, you get a free retest, so you have a chance to correct things and bring it back in a set period of time. That's where the forum can help.
John B.