If publishing is turned on you can just go to the pages document library
and modify the view to show who has the page  checked out.

If publishing is not turned on then pages created through the UI will be
stored in a document library, in which case you can do the same thing.

If the page was created in SharePoint Designer and isn't stored in a
document library but in the site  itself, then you need to open the site
in SharePoint Designer, right click on the page and select properties.
The user who has the page checked  out will be listed on the summary tab
of the properties dialog.