Short of trying before buying, you have to trust the video or product description when they talk about how it looks with or without fog.
I do agree and concede that all the product demonstration videos appear to be shot with fog or at the very least haze. You have to remember, these are product demonstration videos and so they are working to best display the lighting. Naturally, they are going to bias the lights in a manner that best shows the product. Hey, if it was me, I'd probably do the same thing myself.
I will say this, I have an older Mystic and Vertigo, and they look great with and without fog. My Triloy and Double Twist work well without fog as well, but definately work better with fog. My Sunray III works ideally with fog, or at least with a somewhat low ceiling and not too far off the floor with the platform perpendicular to the floor. My scanner-type effects(Rover, Barrel Flex, Reflex, Electra) all work best WITH fog.
With my movers, my Chauvet Q-Spot 150's, those work great with or without fog. In fact sometimes they look better without fog so the beams don't show up, only the gobo at the target.
I won't mention my wash lights as they just don't care, and it's not the purpose. But, I did run my Mega Bar 50's through ground fog and it was freakin' awesome!
Another issue is are you looking for wide coverage?
See if any retailers will hook one up for you before you buy. I know Guitar Center typically has a decent rig of stuff, but it isn't varied enough based on what I saw when I was there last week buying an IEM. It was still a good selection though. Short of that, be patient waiting for results.