Look at the fuel. Is it still clear? The only real contaminant that you should be worrying about is moisture. Methanol is highly hydroscopic, meaning it likes to suck up moisture. When it does, it starts to turn white. If the fuel is still clear, you can use it with no ill effects. You might see some whitish flakes at the bottom of the bottle, if the fuel is really, really old. Shake it up, and it should go away, that is the castor oil congealing a little. If it's synthetic, you shouldn't see that.
Good luck, and remember, nitro fuel doesn't go bad like gasoline does.