What type of warning systems do places have to tell people there are going to be tsunamis?

There must be an earthquake detector in place to detect any earthquakes off the shores in the oceans. This system can be a regional or local system, and although the system is fallible the margin of error is very small.

Sometimes though the earth quake/movement is too close to shore to detect it in time, and people get caught in it as a result.

There also needs to be a universal warning signal that actually tells people that the tsunami is coming.

I hope this helped.