General appearance: does the car appear to have been well cared for? Do you have access to any maintenance records, CarFax, etc? As others said, any rust? Leaks on the ground beneath the car; particularly near the rear main seal? Signs of a serious collision? Repaint, Bondo, broken interior plastic?
These are long-lived and generally durable engines. The transmission will crap out first, and the body will likely be a pile of rust flakes before the engine starts smoking oil. The Optispark distributors can be a bit costly to replace unless you do the labor yourself.
Other than that, its a typical big GM car from the 1990s. No real worries.