check one or a few specific pages

Choose a small number of manual pages you maintain or care about. Run those check tools — mandoc(1) -Tlint, mdoclint(1), igor(1), and maybe gmdiff — you have installed or can easily install on them. Prioritize your findings and take notes about those you consider relevant. If desired, prepare patches to improve the manuals.
Ask somebody who is experienced with mdoc(7) to have a look at your patches, then send them upstream for inclusion.

Don't attempt this with OpenBSD base system manuals, almost no such issues are left in OpenBSD, and those few that remain are very hard to resolve. It is very unlikely that you can come up with any useful patches to the OpenBSD base system in this way.
September 26, 2014