The same way they prove so many other things in space or otherwise before actually seeing it. So, yes, supposedly there wasn't actual picture evidence up close of it until now. However, their previous statements weren't based on what someone "thought" they saw. It was based on many things, ie; soil, elevation, erosion, etc. I really can't remember all the details as the only ice I hunt for is in my freezer. Hmm, maybe they saw some old ice cream containers, too.
If you remember, they also claimed water on Mars, then backtracked. Once confronted with outside groups in refute of no water/ice on Mars, they backtracked once again. There's an awful lot of that going on concerning anything space related for some reason. Conspiracies aside, I guess that's to be expected when you have so many groups/individuals studying things we not only know little about, but have no means of getting "hands-on" with it.