How do you test alternator charge with a digital multimeter?

Check the gauges, if the car is equipped with them. If you have a volt/amp gauge, it will read the alternator output for you. Turn on the blower fan for the AC or Heater, the headlamps, and any other accessories that put a strain on the alternator, and watch the gauge to see if it decreases voltage or amperage. As a rule, if the voltmeter is higher when the engine is running then when the engine is not, you can confidently assume that the alternator is charging. Check it at the battery with the engine running. It will show at least 13.5 volts if the alternator is good. If these values decrease when you turn on electrical accessories, the alternator may be weak. Listen to the alternator while the engine is running. If there is a problem with the bearings you may hear a squealing sound coming from the front of the car, which becomes louder with more electrical accessories using power at the same time. Feel the alternator after the engine has run a few minutes and you turn the engine off. If it is very hot, you may have wear to the bearings or the insulation on the copper windings may be breaking down, an indication the part may fail soon.
To actually measure the charging current on a cheap modern digital volt meter: Set meter to most accurate voltage range, with the engine off measure the voltage between chassis and chassis connection at the battery, this should be nearly 0 volts, now turn the headlights on, the headflights will draw about 10 Amps and the volts drop across battery earth to chassis will show about 50 millivolts dependent on the resistance of the earth strap. Remember 50mV is proportional to 10 Amps. Turn the headlights off and start the engine, the meter will show the charging current and this will be in the opposite polarity as the current will flow in the opposite (charging) direction. for example if the meter shows 100mV the alternator will be charging at 20A. turn the headlights on and you will see the charging current to the battery reduce as half the alternator current goes into the lights.