Flying a machine without Vnav ,FMS etc I tend to do a quick calculation based on the fact that I will go 3 'nautical' miles per 1000 feet of altitude lost in nil wind conditions. With a headwind I won't go as far and with a tail wind I'll go a bit further. This assumes that I will maintain a speed close to .7 Mach or 280 knots depending on altitude. Once established in the descent ,I can monitor whether or not I am going to make the assigned level and make adjustments accordingly.
In the climb it's more difficult as any calculations you make are more sensitive to the changing wind with altitude (your groundspeed) and your weight (your rate of climb). Helps if you've got a computer to help you but you have to do a calculation based on time=Distance/groundspeed and then look at your rate of climb to see if you're going to make it!