What are the warning signs of a tsunami?

Water level changes: There are two things to watch for: a strong earthquake lasting 20 seconds or more near the coast; a noticeable rapid rise or fall in coastal waters.

There may be very: few. This is because a tsunamis may hit thousands of kilometres from the earthquake that caused it. Thus it may not be felt. When the tsunami is in deep oceans - it may not be apparent. Right before it hits, the sea may briefly recede from the coast. However the best way to find out is from a a tsunami warning system which includes a network of tsunami sensors. Not all countries have this.