Well saying that any carrier has their own cell sites is not entirely accurate.
http://cache.gawkerassets.com/assets/images/4/2009/10/500x_Cell_Tower_Visit.jpg
That's a typical cell site. This one has 3 carriers on it. I've seen some towers with all 5 before. Sometimes there are two towers very close with 2 or 3 each.
The reality is that in metro areas (cities and suburbs) most carriers will have similar reception since they all piggyback off of the same towers, but they have their own antennas and equipment. In some cases they share the same back-haul link. In several regions cell sites are actually not owned by any carrier, but are owned and operated by a 3rd party company.
They do this because it's such a huge cost and pain to get a tower built anywhere these days, so if someone has a tower, they all jump on it. Tmobile makes the location of all of their owned towers public.
http://www.t-mobiletowers.com/TowerSearch.aspx
In many cases they will list out the carriers on any given tower.
example in baltimore:
http://www.sitemaster.com/DmsSubscr...iginalFileName=7BAC067DELEV.pdf&Doc_Id=673317
Note that cricket and sprint are on this particular tmobile tower.
I wish all carriers (including cricket) had a similar site showing the towers they own and which other carriers are on it.
Here's another one in Baltimore that has verizon, att, on a tmobile tower.
http://www.sitemaster.com/DmsSubscr...iginalFileName=7BAC227CELEV.pdf&Doc_Id=673683
Should give you a good chuckle thinking of all the carrier fanboys who live near this tower saying that such and such has good reception and no one else does.
Anyways the point is that cricket has their own infrastructure/network in the Baltimore/DC markets. The easiest way to tell what crickets towers actually cover is to look at those markets broadband coverage maps (as opposed to the voice maps).