How do i know if its my battery or alternater that needs replacing????? I have a 2001 ford tarus

the quickest way to check is: w/the engine running disconnect the ground(black) wire from the battery.if the car dies the alternator is not charging.

Many shops will test your battery for free. If you suspect you've got an alternator problem, then be sure you get a good warranty on the battery, just in case. Alternators can be a pain to diagnose because sometimes they test fine when they really aren't.

I am on my fourth Taurus. 191,000 miles on this one and have never replaced the alternater. Take it by PepBoys or AutoZone or O'Reilly's, etc. They will come out, test your battery, and tell you.

Simple ruel of thumb..
Put a volt meter on the battery.
11.8 - 12.2 volts is normal
Start car and watch battery voltage.
If voltage falls below 10V during cranking battery is suspect.
When vehicle is running, battery voltage should be 13.5 to 15 Volts This shows alternator charging.
If voltage is same as step 1, alternator is not working.
Yes, I know there are lots of different things that can change this, but this is easy and will give a good indication most of the time. There is no substitute for a VATS Tester.

Like the others said - have it tested. And do it quick. The alternator is one of the few things that stops you dead in your tracks. And it doesn't give a shit if those tracks are on the highway, in a snowstorm. It'll kill ya.

Finding a mechanic that has a clue can prove to be difficult.
You also have a third thing to check, the regulator/rectifier.
But to answer your q. There's usually 3 wires coming from the genny (alternator), usually they are jellow.
With a meter set to AC, connect one to the + one to the - start engine, it should read some 90V a bit less is ok.
The yellow wires have no polarity so don't worry, go round all of them in this way, make sure there's at least 60V.
If you touch two of the wires together you burn the lot!!
If good, connect it all back, set meter to DC run engine and connect to battery. Rev a bit, it should oscillate a little bit over 13V (might be difficult to see) or rev up and see if lights get brighter.
Now you know if the rectifier works.
if Regulator not working then the battery gets very hot (overcharged) especially on long runs and older types boil water off and dry up. In that case your regulator is shot.
To see if battery (that has all its fluyd in and appears good) is shot just trickle charge it overnight and refit. If no good it won't hold a charge.
That's all there is to it, pity 85% of mechanics are clueless but to be fair, wires have nothing to do with mechanics hence they are excused.
There could also be broken wires in between, so they are also to be checked for continuitivity.
EDIT: if your genny needs replacing there should be a red light that won't go off. One of the ones that are on but come off soon after starting engine. At least we have them on Italian cars, not sure about fords.

Take your car to the repair shoppe.
Try this link http://www.whathappensnext.com

I was driving and came to a red light and my battery light came on then the car died so im guessing alternater huh??