GreatCall Jitterbug coverage used to be provided by a patchwork network made up of a whole slew of carriers. My Jitterbug J originally came with a Qwest sticker on it. However, coverage is now provided by Verizon.
Verizon's network is, of course, very good.
In any case I have personally found the coverage to be very good up and down along the East Coast and in the Mid-West.
There are a couple of ways to know whether you have a good enough signal.
Jitterbug has two way for you to check your signal strength. The quick way is to just flip open the phone and check if there's a dial tone. If there is, then you can make a call. It's that simple.
You can also go to the "Phone Info" section of your menu. There, the actual signal strength will be given as either good, fair, or poor. All these levels still appear to be strong enough to be for making calls. I still haven't seen what it says when it can't make a call
You can check whether you have Jitterbug coverage by entering your zip code on the GreatCall homepage.
While GreatCall uses Verizon, their competitor, Consumer Cellular, relies on AT&T.
Tracfone relies on different providers depending on where you are and which phone you buy. However, this coverage page is based on phones on the AT&T network.
A few other providers rely on Sprint for their coverage. One of the more prominent examples is Virgin Mobile which was eventually bought out by Sprint.