how do I know if my system is really working all night

I have had the system for over a year... I have had one or two sensors replaced as they malfunctioned.  Now, the front door sensor did not work this morning.  It has worked for the entire time I have had the system.  The alarm was on and the front door was opened.  Nothing happened.  I am starting to have another problem with a sensor not being recognized.  I guess you get what you pay for.  I am starting to look around because who knows if any window or door could of been opened last night and the alarm did not work.  It just happened the front door didn't work.  What is concerning is that every sensor might not have been working for me last night!!!!  We don't know as I only happen to open the front door prior to turning off the alarm.  Customers are complaining about his issue and as always, it happens after hours and no one to call when the problem occurred. Everyone assumes this alarm system is working all the time.  Is it?  Is there a way to check to see if it was monitored the entire time?  If the glitch happened today, how do I know it is not happening at other times?