1. They are published on the approach plate, or...
By DME - 300ft per nautical mile (from end of the runway)
EX. Say the MDA = 600 and the runway is 1 DME from the fix VOR, LOC, NDB, ETC) Your VDP = 3 DME 600/300 = 2 DME plus the 1 DME = 3 DME, or...
Take 10% HAT off your time.
EX. Say the published HAT is 800ft and the time for that approach is 5:20 Your VDP will be at 4:00 along the approach. 10% of 800 = 80 seconds, subtract that from 5:20 and you get 4:00