Underground water, or aquifers are broadly classified as 'confined' or 'unconfined'. The latter is open to the atmosphere, so this type would simply evaporate away on Mars, leaving caves or extremely porous, unstable soil, maybe sinkholes. The 'confined' aquifer is capped by a layer of low-permeability rock or clay, and is pressurized by the weight of this confining layer. But there is no significant water on the surface of Mars, so underground aquifers wouldn't form in the first place. There are no bodies of water that would force water at high pressure into a confined underground bottle. If such lakes existed in the past, they don't anymore, and the aquifer contents would be squeezed out by the rock pressure.

The previous existence of large volumes of water would be indicated by caves & mineral deposits (stalagmites), and sinkholes left after evaporation. Water can also crack boulders when it freezes, if it were ever liquid. I would also look at mineral deposits on what appear to be lakes, where the water level would have risen and fallen with the season, leaving a rusty mark on the rocks where the surface level was. I wouldn't trust sedimentation and flow channels as evidence of water because given geologic time periods, many things can create these formations. Even the dust and sediments themselves will act as a fluid and carve out more channels and erode rocks.