"Powers of observation" is the usual expression. This is a collocation, where 'powers' is almost always in the plural. This seems to make sense, as there are so many types of observation, and not everyone will be skilled in each of them: you might be an expert in observing meteorological signs by observing the sky, measuring wind speeds, and checking satellite data etc., but you might be useless at observing your spouse's dissatisfaction with your behavior.