You either see it.... OR right before it develops, the air is very still, the sky might be a strange color. Also there will be a visible funnel cloud beginning to come down to touch the land and then...
One way you can tell is if you watch animals around you. They start to act abnormally. Horses will always have their muzzles in the air, sniffing...etc. Some people have the ability to tell by...
If the tsunami is caused by an earthquake, a slight rumble can be heard, and the ground might shake. As the waves approach shore, the water near the shore will recede dramatically, following a...
Drastic recession of water waves. For example, if you are able to see the ocean floor, where you couldn't see it previously, then its time to head for high ground. Animals are also a good sign,...