Her Mr. Kilmer I have a 1988 Ford f150, when I bought it the man said it did not run. It started with starter fluid then died. I have put a brand new alternator in it. But I think it might be the battery... What should I do before I replace my battery? Thank you.

Take the battery to auto zone, they load test them free. If it's bad, replace it. If not, recharge it and check for vacuum leaks or low fuel pressure