You can download the host test gametype and map, or go into theatre and see which player it says is host. Either way you're not guaranteed to get the same host once the game starts.
You can tell you're host by shooting the barrels and if they blow up instantly you're host. Or if you throw a grenade and it comes out with the animation. Telling if other people are host is much more difficult. I miss halo 2's trick of holding down the back button and whoever spawned first was the host.
I believe Mick is talking about this: YouTube - Halo 3 Tricks: Episode 48 *Who's Host?* This video also shows the way of telling who's host in Forge. Other than that, I don't believe there is any other way of telling who's that hasn't been mentioned.
In forge to tell if your host, look all the way down at the ground, turn to forge, and turn back, if your still looking at the ground your host, if not then your non host.