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, leaving fish, seaweed coral and such like. (following a 'sucking* sound'). A clear rumble should be heard. The fast moving waves will start to get higher as the seafloor gets shallower near the coastline. This forms a 20m wall of waves (a tsunami) and the waves will crash onto shore and rush inland. Sometimes, warnings are given to the country and surrounding countries.
*Note that 'sucking' means being sucked in, and is not a vulgarity.