man-preview

M-x man-preview displays a preview of man nroff source using "man -l".
The best feature is that when re-previewing the same file or buffer the
existing position in the preview is preserved, so if you've just changed
the source a little you should be still quite close to where you were in
the preview, to see how the change has come out.

M-x man "-l filename" does almost the same as this, but it depends on
having a disk copy of a buffer, so it can't work out of tar-mode members
etc.