How do I check my voicemail I forgot password for my cell

Call your Cell provider.
You need his or you forgot yours?

You don't, if you don't trust him, then don't marry him, you should not even be at a point to consider getting married, if you have that serious doubts that you want to go behind his back and hack into his voice mail.

And of course when he finds out what you are doing he would not want to marry you anyway.