Here's what you need to do.
1. Engine off - battery standing voltage.
With the engine off and fully charged battery, hook up the meter directly to the battery terminals(not clamp, bolt, nut, etc...). Read the meter as volts DC. Should be 12.x volts depending upon temperature and battery charge status.
2. Engine running - lo idle - no load charging voltage.
Engine running at idle, all lights, accessories and AC off. Read the meter after voltage stabilizes from starting engine. Should be more than battery standing voltage. Usually 13.x volts, maybe lo 14s.
3. Engine running - hi idle - no load charging voltage.
Engine running at 1500 RPM, all lights, accessories and AC off. Read the meter. Should be 14+ volts.
4. Engine running - full load charging voltage.
Engine running at 2000 RPM, lights on high beams, AC on MAX(blower on high). Read the meter. Should be less than high idle with no load and more than lo idle with no load.
These readings give a good indication of the charging system.