Trust your nose.
Your nose will tell you that something is crappy long before you have nice, solid evidence. A gut feel about a command-line interface (e.g., the compiler that proudly announces “Compilation FINISHED!!!”, as if this was a seldom occurance), or the GUI that has a bunch of buttons kind of labelled similarly, but not really (do you want to Commit, or Save, or Finish, or Exit?), or the book on programming in which the publisher typeset code in a 14-point sans-serif font, with huge line spacing, no tabs and line-wrapping everywhere. Your nose can tell.
This minitribe brought to you by a bad day at work wrestling with something that I knew I should have just bloddy re-written a couple of years ago.