There are two types of validation, one for the model alone and one for the model in relation to its associated P&ID drawing.
The first is straightforward; you are simply checking for disconnected parts, placeholder parts, or property mismatches. The second type requires an understanding of the types of discrepancies that can occur between a P&ID and model and why. 
Plant 3D objects and their P&ID counterparts do not share data even though they may refer to the same logical objects. P&ID data and Plant 3D data is physically separated in the project database. Therefore, changes made to the P&ID drawing are not automatically reflected in the model. However, you can validate the model against a P&ID drawing at any time to find any mismatches that exist between the two and correct them.