The only thing very specific to your thermostat that I've found at Honeywell's website is an operator's manual that, unfortunately for me, appears to be available only in Spanish. If that's helpful, and you didn't get one with the thermostat, it's a 2.4MB .pdf download at CT50 Operating Manual. It focuses on the CT51, which appears to be a little fancier than the CT50, but it covers both; and it does look like it tells you how to check the nameplate(s) on the stuff in the basement to confirm that you have the right thermostat for your setup - if you can read the Español. Even without reading, you can tell quite a bit from the pictures.
The 30V rating for the Honeywell CT50 is a maximum rating, and this thermostat shouldn't care if any lower voltage is used. It would, in fact, probably work for a while at significantly higher voltage, although staying within the ratings is recommended. All it does is open and close a single connection when it's connected in the two wire configuration you're using. If there aren't any sparks or smoke when it turns off, and it doesn't get too hot when it's on, it will work; although outside the ratings it may not work for very long.
The "professional" way to tell if the thermostat is on or off would be to clip a voltmeter on the terminals where it's connected. Your voltmeter should show "thermostat line voltage" (probably about 24V) when the switch is open (furnace off) and near zero when the switch closes (furnace on).
The cover on your new thermostat is a pull-off kind, and the thermostat will work just as well with the cover off as with it on. The CT50 is the common mercury switch type. If you can see the mercury switch, just look at which end of the glass tube the shiny blob is in. If it's in one end and the furnace is off, moving it to the other end (by setting a different temp) should turn the furnace on. You can probably run down, check, and get back upstairs to turn it back down before all the butter melts.
As long as you can turn the furnace on/off by changing the setting on the thermostat, you can be pretty confident that it will do its own thing when you set it and forget it.
I did get an indication elsewher at Honeywell that your "anticipator" screw for the heat turn-on should probably be set at about "8" for hot water systems. Setting the rheostats all the way to zero can let a lot of current through them, which can burn them up. No mention at that source about where the other one should be, but somewhere around "2" or "3" should be safe enough.