When in doubt it's usually safe to assume that Doc is generally perceptive enough to know - and where perception fails him he'll end up hearing about it through the small grapevine they have in this tiny little town - though ordinarily he doesn't make a big deal of peoples' private affairs.

He is curious about this painted garden though. Does it mean Neal is taking up the brush again?

"Do let me know if you need any help," Doc offers. And Doc doesn't make disingenuous offers - he would roll his sleeves up and start pitching in if Neal were to give him a task right now. Of course, he'd like to take a look when it's all finished, but hopefully this is a many hands make light work rather than too many cooks spoiling the broth situation.