Determine whether 5V line has power using 3.3V IC (Atmega 2560) without burning power
simulate this circuit &ndash; Schematic created using CircuitLab

When enable is 0V (from your microcontroller), M2 is off, and therefore M1 is off which means no current flows through R1 and R2.

When enable is 3.3V, M2 turns on, and pulls down M1 which will turn on M1, which allows current to flow through R1 and R2. 

You can monitor the voltage across R2 and see if 5V is there (in this case a reading of 2.5V would indicate 5V). If its 0V, then you don't have 5V. As soon as you have your result, set enable back to 0V so you don't waste any current.

You can scale the resistors to give you whatever current is acceptable for you.

When you select your transistors, just watch out for the turn on voltages.

M2 needs a Vgs(th) such that 3.3V is sufficient to turn it on.
M1 needs a Vgs(th) such that -5V is sufficient to turn it on.