Simple circuit to light a LED only if I have a square wave
The circuit below could be considered to be a monostable multivibrator, as Spehro suggested, but is cruder than most such circuits. Using an IC will give a "clean" result more easily, but this can be made to work well with suitable design and/or playing.
Component values shown are examples only !!!

This would work on input or output with suitable component values.
Loading of the waveform by C1 may need to be considered depending on the nature of the source.

A MOSFET could be used for Q1. On the input side an "on voltage" (Vgsth) lowe enough to accomodate the 3V3 swing would be needed. On the output side allowance for the high voltage would be needed as most MOSFETS are better operated with lower gate-source voltages.







simulate this circuit &ndash; Schematic created using CircuitLab



One gate of a hex Schmitt inverter (eg 74HC14) could be used in place of the transistor - leaving the other 5 inverters for "playing". You could eg have an LED that flashed at a steady rate regardless of input frequency, flashed only initially then steadied, or vice versa or .... . 
The amount of useful, and fun, things that can be done with a package of hex Schmitt inverters is astounding. 

Just for fun - or usefulness

Gate 1 serves the same function as the above transistor circuit.
Gate two provides a flashing LED at a user defined frequency when the square wave is present.  4 square gates invite experiment.  

The LED drive is limited by the gate drive capabilty. For brighter LEDs a transistor buffer can be added. Component values are "indicative". 







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