After several months of dev, a project could easily become ugly, probablly contains many used resources files(like pngs). It is easy to remove images that you know they are not using anymore, but that's not working for these looks unfamiliar. So i need to check their existence, it's simple to do a search in xcode, but xcode doesn't search in the xib files. Here is a line of command you can use in the terminal:
grep -i -r --include=*.xib "text you want to search" /your project's path
This command could search for all files (*.*).