What octane rating of fuel does my 94 Chevy truck require?

What octane rating of fuel does my 94 Chevy truck require? This is my first vehicle. It is a 1994 Chevy c k pickup truck. How do I figure out what octane rating of fuel my truck requires? The last time I put gasoline in it I used the 89 octane rating. I'm not sure if that's the right one.