One measure of efficiency is power output/power input.
If you may have from the input ability to the air compressors for sale in watts, the output flow rate in liters for each minute, and the output pressure in bar(absolute), then the formula for efficiency is
(V * 100 * ln (P))/Input
Where v is flow rate in liters per second referenced to atmospheric pressure, P is pressure in bar, and Input is chance to the compressor in watts. This also assumes which the input air is roughly atmospheric pressure and near room temperature, as well as output air has cooled returning to near 70 degrees.
A rule of thumb is the fact 1 hp to a compressor will generate 4cfm at 100psi. Lets see what efficiency which is.
1 cfm = 1cu ft * 28.3 liter/cu ft * 1 minute/60 second = 0.472 liters per sec
100psig = 115psia = 7.9 bar absolute
4 cfm = 4*0.472 lps = 1.89 lps
1.89 * 100 * log-natural (7.9) = 390 watts of output power
1 horsepower = 746 watts.
390/746 = 52% efficiency