To validate a witness using the online server, go to:

  http://vcloud.sosy-lab.org/webclient/runs/witness_validation
 
Select the witness GraphML file and the .c (or .cil or .i) file
to be validated, and click "Submit run"

If there are no formatting issues, the page will refresh when
the run has completed.  A successful validation will include the
string towards the bottom of the results: 

  Verification result: FALSE.
  
As explained at https://sv-comp.sosy-lab.org/2016/witnesses/ :
"The verification result "FALSE" means that the error-witness was 
successfully validated, i.e., one of the paths that is described 
by the witness automaton leads to a violation of the specification. 
The result "TRUE" would mean that none of the paths described by 
the witness automaton lead to a violation of the specification, or 
in other words, that the witness was rejected. A witness is also 
rejected if the witness validation does not terminate normally.

There is a way to run the validation locally with the latest
version of CPAchecker, but I have not yet been able to get this
working on the MacOSX build of CPAchecker.