On many 1998 5.7L Silverado engines, the PCV valve is located on the driver's side valve cover. It's usually a small, usually black, cap-like valve with a hose attached. It might be somewhat hidden, but it will be near the valve cover itself, and the hose connected to it will be part of the PCV system.
To be certain:
1. Consult your repair manual: A Haynes or Chilton manual for your specific year and engine will have a diagram showing the exact location. This is the most reliable method.
2. Look closely on the driver's side valve cover: Carefully examine the entire valve cover surface and its immediate vicinity for a small, usually black, valve with a hose connected. Trace the hose; it should lead to the intake manifold or a related component of the crankcase ventilation system.
3. Online diagrams: Search online for "1998 Chevy Silverado 5.7L engine diagram" or "1998 Chevy Silverado 5.7L PCV valve location." Be cautious, as some diagrams may be for different years or models.
The location you described on the passenger side valve cover likely deals with the *intake of fresh air* to the crankcase ventilation system, not the *discharge* which is handled by the PCV valve itself. The PCV valve releases positive crankcase pressure into the intake manifold to be burned, preventing oil from being pushed out of seals and reducing emissions.
