Development builds are posted at https://devbuilds.drdteam.org/ecwolf/
I would recommend hanging in the irc channel #ecwolf at irc.esper.net (I think I saw you in there before, but not sure). Between ECWolf being open source and being largely single player focused (thus not very high impact if something goes wrong) there isn't a lot to say about testing besides find issues and report them on the issue tracker on bitbucket.
One of the best things to do is to become really familiar with vanilla Wolf3D and look for instances where ECWolf behavior has noticeably deviated from the original. (For example reports like these
are really helpful in identifying where there may be oversights while some key internals were restructured.)
Another thing to do would be to play though classic maps sets (that don't require modified exes) and check for any compatibility issues. While I don't think there were too many tricks given the map format restrictions, you never know what one might find.