lately every time i turn my lights, or fan on the rpm's drop down. also my car has a little trouble keeping idle and i noticed when it does this the battery meter will go down everytime the rpm's go down a little too. i checked the tbi and it seem to be getting good fuel delevery. could the idle problem be because of my alternator, does it sound like i need a new one?

 

use your multimeter while the car is on, put one probe on the positive on the back of the alternator, and the negative on the battery post. if its less than 16 id say its done.

 

Alts self regulate to about 14.4 volts. Anything under about 12 is suspicious.

Before you write it off, check to make sure both wires to the connector on the side of the alternator have power with the key on.

 

You could get lower voltages because something has changed with the motor - dropping rpm, or the motor could have ignition troubles because of a low voltage.
If the alt loads down the motor then something is bound to get hot (alt or bat). If voltage is lost somewhere it could be a loose connection at the starter or in the bulkhead connector. How low does it drop inside the car?
Do as gurk says and measure the voltages directly - but use Toehead's numbers & advices, although I'd say that under 13.5 is bad and under 12 is disastrous.
Measure with and without load, and vary the rpm in both cases. An alt gets unable to deliver any current at all below a specific rpm, and it needs maybe 1600 rpm to deliver its specified current.