You can do it by the second by watching when the medic spawns and subtracting 40 seconds from the total time left in the map, or you can compare it to how much uber you have currently. The latter method doesn't work if you're dead while the medic is still up, you can compensate by seeing how long you are dead for. For example, if the medic spawns while you're still dead, and you spawn in 10 seconds, then they will have 25% advantage.
40 seconds = building uber as fast as possible
30 seconds = building kritz as fast as possible
The total maptime left method is probably the safest, but I don't always use that. It depends on a handful of situations and what's going on. Do what works for you, as long as you have a really good idea of where your uber is vs their uber.