Going to hdradio.com and doing a search for Boston area is a good source of info though not always current. Recently of course WBIX 1260 debuted and it's said they no longer broadcast in HD (the guide still says Radio Disney, your music your way, WMKI). 1430 WKOX may still well broadcast in HD, or do they? (I only have the FM-HD portable) but is now talk, no longer Mia. I mentioned these on the HD Radio facebook page. Otherwise the guide is probably pretty accurate.
Some station webpages have mentioned their HD services--for awhile at least WKLB had a link to their HD2 (now The Wolf) and when they were WTKK, 96.9's page had a link to
Irish 96.9. Some stations mention their HD status in their IDs or somehow... WRKO's ID says "WRKO Boston, WEEI HD2 Lawrence-Boston". Technically they are on the HD2 of
_WEEI-FM_; WEEI is a Boston-licensed station at 850 which doesn't broadcast in HD)