First things first. If the ends of the wires from the alternator are burnt from an overheated rectifier/regulator you need to fix that first so you are getting a good reading.
Then - if you have good hands and thin terminals at the end of the wires from your multimeter you can do it by sliding the plug back a little on the rectifier terminals. If not, pull the plug off and stick the multimeter terminals into the plug, one on each of the outside connectors. That's where you should have 30 volts or better AC.
To test the rectifier/regulator, you probably need to check the positive and negative battery terminals, again with the engine running. You should have around 14.5 volts DC there.