How do we know that water used to be on Mars?

Answer 1

Valleys and river beds can be spotted from orbiting satellites, indicating that a liquid one flowed. A recent picture actually shows features that have changed over the last few years only, so there could still be some water flowing today.
Of course, that could have been other liquids doing that, but most other liquid's presence is even harder to explain on Mars than water.
So water is the most likely culprit.

Answer 2

We don't know, we guess.

There are what appear to be rivers, channels, dry lake beds and every indication that we would expect to see on a planet that formerly had liquid water.

Also, there is known to be water on Mars, but in ice form.

It was recently discovered that Mars may even still have a small amount of liquid water to this day. We know this from comparing pictures we've taken of these channels. Though water was not pictured, comparison shows that objects in these cannals have moved and moved in shuch a way that they seem to have flowed around objects that they would not have been able to unless they were floating in a liquid medium.

There may be other explinations for all of this, but the most logical is 'Water', so far.

Answer 3

NASA's Opportunity rover has found convincing evidence that large quantities of water were once present in at least one location on Mars. "The rocks here were once soaked in liquid water," said Steve Squyres, principle investigator for the Mars Exploration Rover (MER) mission, referring to the bedrock outcrop near the rover's landing site in Meridiani Planum.
Evidence suggests that, at some point in Mars's past, water was present in sufficient quantity to make the region "capable of supporting life as we know it."

Confirmation of water's role came from a series of detailed measurements made over the past few days at El Capitan, a small section of the rock outcrop. Both microscopic images and spectral measurements, which can reveal specific chemicals and minerals, helped to convince scientists of water's historical role.

Squyres did, however, offer a caveat. While the outcrop was "definitively" altered by water percolating through it, he said, scientists are still not certain whether water played a role in its initial formation.

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Answer 4

Because we know that water is there now, to the tune of enough to cover the planet 100 meters deep, or deeper. Also there are dry river beds, gullies, clouds, snow etc. etc.

Answer 5

because of the polar ice caps. ice=frozen water
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O that and the fact that there are water pattern erosions on the surface of mars.

Answer 6

the marks of flowing rivers and frozen ice caps.also some traces of fossils indicate life existed which would not be possible without water.

Answer 7

It has what look like channels or rivers that could have carried water.