Those lenses have very little overlap, so you will generally known which one you need. They are not interchangeable for any given subject. There are subjects you can take with both but the result will not be the same except between 40 and 42mm, so the one you use is the one that gives the results you are looking for.

The 14-42mm is a wide-angle to medium lens. It is good to shoot things that are relatively large such as buildings, interiors, etc. You can rarely use a longer lens for such subject because you would need to back away which is often not possible. Imagine take a shot of a large temple, unless is is in reserve or park, you can rarely move far enough to see it all with a long lens.

The 40-150mm is a telephoto lens and shows a smaller angle-of-view. It is used to pick out details, such as a person's face for a portrait, as opposed to a full-body image which is easier with the wider lens. Again, many times you cannot compensate by moving yourself closer to the subject. Doing so for a person also results in an unflattering perspective which is why portraits are rarely shot with a wide-angle lens.

The only thing to minimize lens changes is to buy a Zuiko 14-150mm which covers the whole range of both your lenses but you will lose some image quality by doing so. I've covered this choice explicitly in my M.Zuiko lens round-up article.