I hope someone posts a better solution but here's what I do.
I turn on my poly count display. Just click on the display menu and select Heads Up Display and then select poly count. This will turn on the poly count, probably in the upper left corner of your view.

Then I select dragging around verts to see if the vert count (third column) is 1 or if it's more. This can be time consuming unless you know the general area where there might be unmerged verts.

This is the best help I can offer so I hope someone else has a better idea. Perhaps with "clean up" but I don't know. I've still got tons to learn.