nesan_buhk
vacuum gauge

sonny_too_much
You can use orifice plates or a Pitot tube or a turbine meter. Be very careful about restricting the flow into a compressor. Catastrophic failure will occur.

oil field trash
An orifice plate will work but in general it is not a good idea to restrict the flow to the suction of a compressors because it reduces the performance. Is this a centrifugal or reciprocating compressor? On centrifugal compressors as a part of the surge control system it is not uncommon to use a pitot tube device to see changes in flow rate. A pitot tube has very little pressure drop and since you want acfm on the suction for surge control, it makes sense to measure the flow at the suction rather than the discharge. For reciprocating compressors there is very little reason to measure flow on the suction. It is typically measured on the discharge where the pressure is higher and pressure loss from the orifice plate is not as significant.