My car wont start, how do i know if its the batterie or the alternator thats the problem?

If it's the battery you'll know straight away because the starter motor won't run. If the battery's OK but you're not getting ignition, there's no easy answer - there are all sorts of problems that might cause that to happen. The best thing to do is consult a mechanic.

It's in my mind right now because my car is having all sorts of problems - sometimes it won't start, sometimes the engine splutters and cuts out for no reason, and then won't start. In my case I've got to shell out for a new distributor. In your case it could be any one of a number of things. Find a good garage or dealership, and give them a call.
Check the lights or radio its the battery if they won't go on. I think !!
call the AA or RAC
If it cranks? Its NOT the battery.(it should crank rapidly)
If it cranks slowly you need to charge or change you battery
Check to see if the battery terminals are loose or corroded. (Twist them see if they move) They shouldn't.

If it's the alternator. Just replace the alternator you don't need to replace the battery. The alternator will charge your current battery.
when u turn the key, does it make the startup sound? if yes, its just the battery..
if not, then its the alternator
First change the battery, and then if it quits starting again, you will know it is the alternator

either way your going to have to buy one or the other, so start off with the battery, because if it is the alternator, you will probably be better off buying a new battery also

Then again, it could be out of gas.
If it coughs a bit and then dies, it's probably the battery.

If it just clicks when you turn the key, it's probably the alternator.
check the connections on the battery. put the key in and turn till all dashboard lights come on. dont start it tho. then try radio, heaters and window wipers etc.

listen for the engine turning over too. if it isnt turning over then yes it could be the battery. if it is, but slowly then could be the battery. try bump starting it.

if it is damp or has been raining/snowing try some wd40 on the ht leads and spark plugs. this sometimes dispells the moist that can disrupt the current from the alternator the the plugs.

otherwise call a garage to help ya.
change battery first its the cheapest of the two. still if no joy then go for a recon alternator as they are half the price, you will need the old one as this is a exchange for the recon.
don't know where you live but here. you can get both checked for free at some auto parts stores.
go see a mechanic. dont go by these ( the worst i have ever seen here, sorry) answers. it doesnt cost much to have a battery and alternator tested. geez.
Get another car to try to jump start your car. If your car can start and run with a jump start, it is the battery that is bad. If you can't jump start your car, you probably still have a bad battery (maybe with a shorted cell that won't allow enough power through to jump start), but you won't be able to tell about your alternator until you replace the battery. Most shops will hook up a tester to check battery and alternator if you can get the car to the shop.
it could even be the starter motor, need more details!