my 2001 toyota tacoma wont start,battery 2 yrs old.How do I know if its the Alternator, battery or something else?

First check your cables for corrosion and make sure everything is tight. Even down at the starter. Buy a volt meter. Put it on the battery with the engine off. See what the battery has for volts. If less than 11.5 replace the battery. When engine is running, put the volt meter on the hot side of the alternator. It should be putting out 14 volts. If not, you have a bad alternator which is killing your battery. Check out vid... https://www.youtube.com/watch?v=fpPOcO6f798

Take it to auto battery and electric. They will analyize your car and tell you whats wrong for 10.00

Most big box auto stores autozone , oreilly will check the charging system for free! First you have to get it started! Make sure battery posts are clean of corrosion, all electrical contact points are clean and grounded. After battery alternator look at spark plugs and coils! If you jump start you can do secondary check of alternator for output of 14 volts, static battery , as mentioned , indicates discharged or damaged battery! If you decided to drive it down have someone follow you with jumper cables in case it dies again! While at big box auto store have them connect OBDII SCANNER another free service to see if it is giving a code ( failing coil for example) if you found this useful please click the helpful link