Well the very thing about the ideomotor effect is that it is an unconscious action, so no, you wouldn't be able to 'easily tell the difference if you're being observant'. Also in that Wiki page there is this paragraph:
"Scientific tests by the English scientist Michael Faraday, the French chemist Michel Eugène Chevreul, and the American psychologists William James and Ray Hyman have demonstrated that many phenomena attributed to spiritual or paranormal forces, or to mysterious "energies," are actually due to ideomotor action. Furthermore, these tests demonstrate that "honest, intelligent people can unconsciously engage in muscular activity that is consistent with their expectations".[2] They also show that suggestions that can guide behavior can be given by subtle clues (Hyman 1977)."
I didn't link to the Wiki, just a Google search, there is plenty more about the Ideomotor effect online if you want to read about it.
Also, as you haven't said what experience you actually had after the ouija board it's pretty difficult to suggest how it might have happened, most things can be explained through confirmation bias though:
skeptic confirmation bias - Google Search
