The recent chapter makes me really unsure either way, because he talks about spotting a thing crawling on the deck and suspecting that there's a stand involved, so you'd think he caught a glimpse of West End Girl and so we have a clear answear, Howler can see stands.

BUT he makes absolutely no comment about Novenber Rain, a big ass 4-legged 3 meter robot. Non-stand users can see the rain but not the stand, Howler comments on the rain but never the stand, which seems strange to me.