There should be a little light on top, by the knob. If you nudge the knob just a hair to increase the temp, (on mine that's clockwise) the light should come on.
Here's what I have found with mine: I set it up, get the temp stable, which takes about 24 hours. Once it's right, I wait another 8 hours or so to make sure it'll stay put. Then I add the eggs. No water. (I dry incubate, until day 18, then add a little water to raise humidity to 55-60%.)
Once eggs are in, the temp will drop a little. DO NOT ADJUST AT THIS TIME. Just leave it alone, and let the eggs warm up. This may take as long as 24 hours. That's ok. They'd warm up slowly under a hen, too. Start counting the 21 days, after the first 24 hrs.
If they are below 99.5 at 24 hrs., wait another 8. Recheck. If they're still a little cool, give a very slight nudge up on the temp. As slight as you can manage. Then wait at least 2 hours, and recheck. If it's right at 99.5, recheck in 8 hrs or so. I get a slight fluctuation with mine, it'll run about 99.8-100.5, and that's perfect for mine. Any lower I get late hatches, any higher I don't want to try. At this temp, they hatch right on schedule, which to me means the temp is right.
If you fall into adjusting the temps constantly, you'll get spikes and drops, very bad for the eggs. Wait and see, always, before you mess with it, UNLESS TEMP IS TOO HIGH. They turn it down a hair, and open the top for a minute to cool it off. Recheck every 15 minutes until its low enough to be safe. Eggs tolerate cooling a little pretty well, but you can't un-cook an embryo. A little cool, they can make it. Too hot will kill, a lot faster than too cool.