How could I know if there is Tsunami coming?

The truth is you can't know if there is a tsunami coming as seismologists are still unable to identify the exact factors which would pinpoint a wave, much as they are still unable to pinpoint when an earthquake might hit. However there are warnings that could be looked for;

1) foremost on this is if there have been major earthquakes in the area.

A good place to keep up to date on tsunami warnings in the regions of the Pacific and Indian Oceans is the Pacific Tsunami Warning Center http://ptwc.weather.gov/ maintained by the National Oceanic and Atmospheric Administration of the United States of America.

The site gives us earth quake occurrences and readings across the globe and also the severity of the occurrence and the likelihood of a tsunami.

It also goes on to further to offer the estimated time that a wave could be expected at a place if in fact there is a tsunami.

All times are expressed in Zulu time that is GMT +0000; so would need to be converted to the time zones of the local regions.

2) Receding Sea Water / Tides often up to about 1 to 2 miles into the sea, you might notice fish flopping belly up in the ocean. This is often a sure fire way to tell that there is a Tsunami on its way. This was something that was noticed in the 2004 Boxing Day Tsunami.

3) You might also notice flocks of birds flying inland, and even animals moving further inland; it seems God has given these animals inbuilt seismic meters :)