For example, if you see what looks like one of your Tesla Troopers coming toward your base when all your Tesla Troopers are charging T-Coils.
Also, look for units which you know you only have one of, or that you know are doing something else that are going toward your base. Look for units that are not behaving the way they should, like Chrono Legionnaires which are walking instead of teleporting, Tanyas which go around water rather than through it, or Rocketeers which are earthbound. Dogs help, but in the event they are killed or you do not want to use them, try the following strategy. Spies can be exceedingly annoying, yet are easily discovered.
