If you don't have one get a cheap volt meter form Harbor Freight like $4
Test across the battery as it is and then disconnect one terminal and test across your battery. Then test from the battery to the disconnected current.
Then test your battery hooked up and the engine running.
If the batt volts are the same hooked up and unhooked it is your battery or alternator.
If not there is a draw and the batt to cable voltage can help lead you to it.
If no change then +change with engine running its battery
if no change with engine running, bad alternator.
This should take you all of 15 minutes.
Get back with the results and we can help narrow it down.