If you can plot IF Power (out) vs RF Power (in) then that should be enough. Best done as a swept simulation. At lower input power levels the response should be linear so 1dB extra input RF Power gives 1dB extra IF Power. Imagine that line extending on infinitely. Then at higher input power levels the output power level should start to drop below that line. Find the point where the difference is 1dB will indicate the 1dB compression point referred to either the input power (x axis) or the output power (y axis) as required.
Plotting (IF Power - RF Power) vs RF Power and look for 1dB roll-off would give the same measurement. This will give you the gain at 1dB compression or input RF Power at 1dB compression again.