Character is "talking to" someone who is not on stage (dead) how can actor convey to audience that he is "talking out load to this person".

Thanks for advice.

Well, it would depend on the situation but if the character was 'talking' to no-one in particular I think the audience would understand.

I know this is a stupidly simple answer, but wouldn't few looks 'heavenwards' during the dialogue indicate to the audience where the other person might be.....

Okay, so I'm stupidly simple!

I would also go with looking heavenward. It tends to make things a little clearer, though it really does depend on the context of your play.

Good luck whatever it is though

May I suggest that you think back to times when you've talked (out loud) to yourself? Or is it just me that does this.... first sign and all that... sigh. Seriously though, you tend to look down or into far distance. Try both.

Make your partner lie on the sofa pretending to be dead, then walk around the room moaning at them. Then think back - where did you look? That's what to do on stage then.