My general rule is 10 amps for a normal receiver. Anything more and you better look to a power distribution option of some sort. To monitor this amperage you can easily add a 40 amp sensor to the receiver battery (or BEC output). You want to see the maximum the servos will draw, not idle. A good servo for a modern glider for example can easily draw 2 amp on its own, but they really should not, properly setup you should never be near the stall torque of the servo. In other words, make sure you have setup the radio correctly so your servos are not running into mechanical limits of the flight surfaces, etc. For a glider, our biggest draw tends to be throwing in a full crow mix while still carrying some speed. Even that, a max amp reading should only last a short period of time.
At a recent large scale gas event I was surprised to see many planes had no power distribution systems.