There is no real evidence of a Martian civilization. But that doesn't meant that there ISN'T one yet to be discovered. There are three things to consider when examining this topic: 1) Was life on Mars at one time possible? 2) Is there any evidence that might indicate a civilization on Mars? 3) What do we need to do in order to prove conclusively that there is, or was once a civilization on Mars?
Was life on Mars at one time possible?
There is substantial, but not conclusive evidence that life once existed on Mars. Studies of ALH84001 Mars meteorite indicate the presence of microbes. This is not conclusive, but strongly suggests the existence of life on Mars. Satelite photos show dark spots on martian dunes that might indicate the presence of extremophiles. Liquid water is a necessity for loife, according to our best theories. Water obviously existed on Mars. We can see the effects of water erosion. The Mars Global Surveyor showed evidence that water may flow on the surface of Mars, even today.
Methane and ammonia have been shown to exist in the martian atmosphere. Since neither can exist for any length of time in the present conditions, it is suggested that it is being produced constantly by extremophiles. The Europeans have shown that much more formaldehyde exists than was expected, suggesting microbeal life.
The existence of silica on the surface indicates that at one time the environemtn was suitable for microbeal life.
Is there any evidence that might indicate a civilization on Mars?
The evidence of intelligent life building anything on mars is very sketchy, based usually on faulty analyses of photographic evidence. The low resolution pictures that showed a face on Mars, or indications of a complex of buildings and pyramids in the Cydonian plains of Mars, were shown to be no such thing when higher resolution photographs of the same region were examined.
But there are still enigmatic photos that are intriguing. There are a number of "glass worm" tunnels that NASA claims are dune trains, a natural phenomena. But their own comparison photos of actual dune trains and these features are not anything alike. They could conceivably be underground tunnels, manufactured by intelligent beings and uncovered through erosion. Not likely, but conceivable.
There is another artifact called "the port." It resembles a very symmetrical and geometrically complex building. It appears to have two stories, a circular landing pad, doors and windows, and a U-shaped conduit.
A 6.3 kilometers high tower structure can be seen that looks obviously manufacturesd.
Now these artifacts are visible only in photographs, and are merely anomolies until better evidence can be obtained, or these photographs can be substantiated.
What do we need to do in order to prove conclusively that there is, or was once a civilization on Mars?
There are only two real possibilities that can confirm or deny the existence of any life on Mars, and especially if a civilization was ever extant. Human beings from Earth must send a manned mission to Mars. If these Manned missions can set up stable colonies and bases, then legitimate scientists, including eventuall;y archeologists and anthropologists, and exothologists if the need is shown, can exaimne these anomalies first hand.
Robotic missions to Mars are less likely to find a civilization, but more advanced robots could significantly test the surface of Mars, perhaps even dig beneath the surface, to validate the theories of microbeal life existing on Mars.
Both of these possibilities require a substantial commitment to Mars, and that becomes increasingly unlikely as we become more isolationist and comfort conscious. But I strongly feel that the risks and the sacrifices for such missions are more than justified by the possibilities of discovering life on Mars, the need to plant humanity beyond the Earth, and the necessity to discover new frontiers.