A similar suggestion has already been suggested in the topic for the latest update but went relatively unnoticed. Now that things decay when outdoors I think its important that there be an easy way to tell what needs repairing at a glance. Currently we need to try and repair everything one after another and this is more than tedious. I can however understand that the idea of creating new graphics for the levels of decay might not please Jorb and so I suggest that while the cursor is in repair/destroy mode it displays a tooltip with the health % while over an object.
Edit: Seems Rift made a very similar suggestion to mine while I was writing this one.