Well my Airbus draws a nice 'level off' symbol on the map at top of climb or descent so you can get a quick idea from that and your knowledge of aircraft performance. Our FMC also has a Progress page which will give you time and distance predictions for vertical changes, comparing both normal and expedite modes. If you want to be really fancy you can enter a PBD (Place-Bearing-Distance) waypoint in the flight plan, attach a level constraint and let the FMC do it for you exactly. General practice on being level by a certain time is to calculate roughly what V/S you need, work in a fudge factor for the time spent level-ish at each end then climb or descend at that rate.