Volumetric efficiency is how well the cylinders are being filled with air and is given in a percentage. If you take the known total cubic inches of the engine and you are able to fill all the cylinders full when the engine is running, then you 1.0 or 100% volumetric efficiency. This can be measured on a engine dyno where you are looking at the air flow going into an engine at a given rpm. Most local racers and dyno shops do not measure this as they are more concerned about power and torque curves. Some things that do effect the volumetric efficiency depends upon throttle opening and engine speed as well as induction and exhaust system layout, port size and valve timing and opening duration, turbo or naturally aspirated; pretty much anything that can affect the air getting into the cylinder.
As a given rule of thumb naturally aspirated 4 cycle engines are going to fall in the 0.7 to 0.8 range or 70 to 80 percent. A turbo charged engine can have numbers over 100% because the pressure is pushing in more air then the engine cylinder volume. These numbers are not steady and change as engine rpm and load change. If you find an engine that was dyno'ed, the value given is peak volumetric efficiency because the actual efficiency changes and would be on a curve like the power and engine torque are. As a side note: peak volumetric efficiency on a naturally aspirated engine engine always occurs at the same rpm as engines peak torque value. If you had an engine that would run at a steady rpm, under a constant steady load, then you could get a given value, for a given engine, for volumetric efficiency. It is a moving number and as stated above will most likely be in the 70 to 80 percent range.