can anyone tell me if there's a way to test the alternator in my L reg disco.When driving the battery light flashes on and off someone tod me it could be the battery but wanna test the alternator before i go and get a new one

just got my first landrover its a discovery tdi 1993 iam having problems starting but when running it wont tick over the revs keep rising then falling and eventually cuts out any one have any ideas