You can call Verizon technical support and maybe you'll get a knowledgeable person who knows exactly what it is. More often than not you will receive a clueless answer.
Here's how to determine it for yourself.
Open up the options menu (press CTRL-D and enter the password diagvzw), click on "Field Test" (you may have to be disconnected from the internet) ... and look at the channel number:
Now compare that number to this frequency chart:
A good rule of thumb: If the channel number cannot be evenly divisible by 25 - you are 800 MHz
With Sprint, all of it is 1900 MHz so there is no question.
There is a nationwide map that shows 850 MHz only (no PCS/1900 MHZ). Go here and select your region.
Factoid: It takes twice as many cell towers to cover the same area with 1900 MHz as it does with 850 MHz ... how many Verizon towers that are nearby may offer a clue to frequency used. For example, in my area it takes five Sprint towers to cover the same area as Verizon does with only two. Using a Verizon cellphone and checking its field test menu will probably tell you as well. wpsantennas.com has a PDF of phone codes here.