Does my CPAP have a way to tell how many hours its been used? I see no meters or smart cards visible.

The 420G has a total hours display. I don't have a manual available to look up the key sequence; it is available from the front panel using normal patient menus.

So, can I look up the days and hours its been used on the front digital display?

You can find the number of hours that it has been running and the number of hours that it has actually been used.

http://www.directhomemedical.com/pdf/goodknight-420g-cpap-manual.pdf

Page 14

Press the Information Access button for one second.

Each time you press the Information Access button you will cycle through the various data display.  
After pressing it a few times, the  display will show an hour glass symbol, a number and an "h" for the total machine hours.   Pressing it again will display an hour glass symbol, a number, a stick figure of a person, and an "h" for the number of hours that it has been used (compliance).

I just did this and the hours of operation are different than the compliance. How does it know the difference of useage and compliance?

The total hours are the total hours that the machine is running; whether you are wearing a mask or not.  The usage or compliance time is the number of hours that the machine figures that you are wearing your mask while the machine is running.    The compliance hours can never be larger than the total hours, and it is likely that compliance hours will be perhaps 10% lower than total hours -- your percentage may vary considerably.

Okay, that makes sense as mine has 130 hrs an 96 hours of compliance. I have had it since late February, but cant get many hours a night, if I am lucky and the weather is nice( as I get stuffy here in Hawaii,no AC) with it as I am on my second mask. Went from a nose mask to full face. Still not fully working.