I grew up with a guy who had younger sisters who were twins. I would swear they were identical, but they were told they weren't. About a year ago, one was diagnosed with cancer and they all had matching testing done. Turns out they were identical. (very sad, actually. The one with cancer passed away shortly before Christmas. sigh...)
My boy and girl are 17 months apart. DD (the younger one) was always very tall for her age, so we were always getting asked if they were twins. I told DH it would be easier to just say "yes", rather than explain why they were so close in size. Nobody ever asked if they were identical, thank goodness. That kind of thing drives me crazy. Boy-girl twins are obviously fraternal, if you have any idea what the word means.
If I had twins, I'd be curious enough to have the testing done.