The HD channels start at channel 500. Channels below 500 are SD. Refer to your Verizon FiOS lineup.
If you are a new FiOS TV subscriber, then you need "HD Extreme" for cable HD channels. The "Essentials" package only includes the local channels in HD.
If you upgrade to HD Extreme, be sure to ask for the Verizon Flex bundle. Verizon offers "Flex" bundle with 20/5 Internet and HD Extreme for $79.99/mo (plus any STBs or DVRs you may have).
keep in mind that the quality of the picture is determined by the network, not FIOS. For example, on ESPN, the Sports games and Sportscenter broadcast in HD, while other shows like PTI and Around the Horn are not. Thus, you won't get 1080i on those programs.
While the show may not be in HD, the HD channel version is transmitted at an upconverted resolution (from the originator) . Also, some of the HD channels only provide 720p (ABC, ESPN, and others) but your STB will output at whatever resolution you select - even if that requires the STB to upconvert (there is no pass-thru mode on the Motorola boxes).
Menu -> Settings -> Video Settings -> Video Format... should be set to 1080i. SD override to whatever you prefer.
Then change channel to something in HD (Channel 600 is CNN-HD for example). TV should now be displaying 1080i.