When would you use the combined gas law as opposed to the ideal gas law?
Given two pressures, volumes, or temperatures and asked for an unknown pressure, volume, or temp.
Whenever it gives you conditions for one gas, and asks for conditions of another gas, you're most likely going to use this Law.
The question may also say "at constant temperature", but will still give you two pressures or two volumes. Just rewrite the equation like so:
It may explicitly say "An ideal gas" or it may give you moles. When given moles and pressure, volume, or temperature, use the Ideal Gas Law.