There cannot be any credible evidence at the moment.
There are literally hundreds of scientists who are convinced that extraterrestrial life must exist, and expensive missions have been performed and are planned to find proof that life exists outside of Earth.
This is to say that there is no preconceived hostility from the scientific community to accept evidence of extraterrestrial life - including extraterrestrial life hosted in Area 51.
If there were any evidence, it would arguably be one of the biggest scientific discoveries of all time - nobody would keep it secret!
What should be remembered that extraterrestrial life is an extremely exceptional claim - as such people need to be extremely skeptical of any evidence presented.
To give you a current example, see http://blogs.discovermagazine.com/badastronomy/2011/03/07/followup-thoughts-on-the-meteorite-fossils-claim/ which is a very good example of how high the bar is set for this type of claims!