How can we detect water on Mars-like exoplanets?

It would be difficult to detect the water-by-weight in the dust of another planet to the precision that has been done for the Martian soil.

However, having said that, there are a couple of ways to detect the presence of water, and that would be, according to "60 Billion Alien Planets Could Support Life, Study Suggests" (Gannon, 2013), to detect the presence of clouds.

To confirm that the clouds are water clouds, not something the iron-vapour etc, according to web article "Hunting for Water on ExoPlanets" (bolding mine),

Using ESO’s Very Large Telescope (VLT), a team of astronomers have been able to detect the telltale spectral fingerprint of water molecules in the atmosphere of a planet in orbit around another star. The discovery endorses a new technique that will let astronomers efficiently search for water on hundreds of worlds without the need for space-based telescopes.