You won't normally fly approaches "out of the blue" so you will either follow a published approach (STAR) or you will follow the ATC instructions. In AFS2 there's no ATC so you will check the altitudes on the chart to the IAF to intercept the final (loc and glide) if the runway features an ILS.
The issue is more from TOD to the STAR, if you happen to fly over a mountain range. In that case you should check the altitudes on a map (or use a flight planner) to see the clearances. That's where the terrain map on the ND is useful as it shows an overlay of the relief in the Now, unless you fly over Himalaya, the Alps or any other well known mountain range you have some margin from FL 370 so I won't be too concerned to start the descent until FL200 and then you can check where you are before joining the initial point in the STAR
