Joe in large measure this is going to depend on what equipment you will use. A handheld with a duck is simply not going to cut it; to really be successful an outdoor antenna - even a simple ground plane - is almost a must.
I've unfortunately heard or read of folks using their PRO96s/2096s for this - while the 138-144 mhz range is well within it's capabilities (and this is where many Air National Guard freqs are found), the 225-400 Mhz UHF band isn't. Even with Win96 opening the radio up, it's simply not designed to work there; consequently you might get a radio that works OK, or is deaf as a post.
Assuming you have a good milair radio and a nice high antenna, here are just a few suggestions as to where to tune...
Ct. Military on Scan Cape Cod Wiki; (these freqs come from a web page with a fair amount of old data, so don't put too much weight on what you might hear...)
We've had Ct ANG units in the DC area, but I suspect the freqs they used were for a tac purpose in our area, and probably not in general use. I would jump on the ConnScan Yahoo group, the Ct.Forum here and/or Milcom on qth.net to see what other targets you might have from that group.
in addition, 3 new freqs have been identified by TinEar and the team over on the Maryland milair sticky;
292.15 ZNY new freq unknown use
282.65 ZNY new freq unknown use
327.60 ZNY new freq unknown use (this one is listed as being in use in the AR205 refueling area around Vt/NH)
Not to mention JFK, LaGuardia, Newark, Hartford UHF milair. Unfortunately the RRDB is weak on these points; you can check the well known N2NOV website (for the NY area airports) and/or AirNav.