The best indicator for me is how much scatter I see when the sun is about an hour away from setting. If the western sky behind my tree line is an obvious rich blue color, I know I'm going to have a great night. If it has a dull blue, almost pewter-like color, I'll have a good, but not great night. If it's almost white-washed and doesn't turn blue again until you're 90 degrees away from the sun, it's going to be a poor night (though typically the seeing will be very good on those nights).
This is my indicator as well. If I can see M33 and M92 with the naked eye without much difficulty I know I have a good night.
Transparency is a tricky thing. Sometimes the lower levels of that atmosphere are crystal clear, and I can look 30-50 miles across the western arm of Lake Superior and see no haze, while up high smoke from distant wildfires makes the sky a milky blue and the sunset red.
I've seen the opposite true as well while observing from over 10,000 feet in the Rockies. Smoke and haze down low, transparent above.
With a truly transparent atmosphere, altitude matters very little. A friend of mine spent a few days on a boat off Indonesia last month and had one of the best views of the Milky Way he's ever seen--and we both live under Bortle 3 skies.