I would ask that users append their comments with a link to the correct site to ask on, as this is still useful information for people and leaves a good impression too.
Absolutely agree with that, although please keep in mind that in some cases the "correct site" could be debatable. For instance, Cygwin is used under Windows hence may seem more suitable for Super User site, but it could be more suitable for Unix & Linux site (and yes, Cygwin is on topic there). Sometimes you have to stop and think whether or not what you consider a best fit would actually fit the user.
You also have to consider the context. If there was conversation going on within the comments for some time, and an established user already questioned the new user on which OS they're using or which tools, comments such as This is off-topic on AskUbuntu can be fine, although I agree that adding a link to that would be more useful.
On the technical side of things, I would recommend maintaining a text file with commonly used comments and learn from top users in our community. Such users as Fabby, terdon, Zanna,steeldriver typically provide useful and detailed comments, so I would recommend copying some of their comments. Alternatively, use a clipboard manager to insert the commonly used comments into clipboard and paste it right away.
I am currently working on one like that,indicator-bulletin, which will soon have ability to have list of commonly used phrases and insert them into clipboard. Hope the community can benefit from it.