You can tell whether this will be available when you load the approach out of the data base - at least one leg to the RWxxx way point will have a slope written in, 3% or similar. Some approaches have two legs, some three. For two legs "on approach" logic starts 2nm prior to that way point, for three at the first way point with the slope written beside.
It is a VERY good idea to have rough speeds entered for these legs, otherwise if anything changes or you go Vnav Path too early the aircraft obediently accelerates to the speed on the descent page, often 240Kts - oops, bugger.
My mob uses V/S until at the point where on approach logic starts. We tend to use the 1000 auto call to remind us to set the MA alt in the MCP.