You can go to
www.uspto.gov and search to see if your group has your letters, crest, etc. trademarked. Pi Kappa Phi does not, and doesn't appear to have even dead trademarks that weren't renewed. So in reality, any group can call themselves Pi Kappa Phi and that's fine. As for the other Delta Phi Epsilon, they are an international service fraternity, and the reason it's ok for them to do that, is because we don't have our letters/name trademarked yet.