How do you determine whether you can us LNAV/VNAV minima on an RNAV approach?
The STC documentation provided with the GPS / FMS installation will indicate the type of minima it can go down to.  

Typically, GA aircraft like a Garmin 430/530 will only be certified down to LNAV minima.

Business type aircraft with FMS's may be certified to LNAV/VNAV minima.  These installations use a barometric altitude input to calculate a pseudo glidepath to be displayed.