How to check a car's ENGINE air filter

This is a post I penned in 2010...but it's still relevant and interesting...I think... :-) 
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Imagine this, a young(er) chick-engineer is sitting in one of her first training sessions post-college.  The piece of paper saying you now have a college education is sitting at home, in the frame your parents bought, in a box...somewhere in her new apartment.  Something is shared with her and it dawns on her, this is probably the first piece of useful/real-world information she has learned in the last 5 years - You CAN NOT tell if an engine's air filter needs to be changed by looking at it!  But wait, she thinks, how many times has she seen or heard at the quickie-lube the technician come to the customer waiting room with a dirty air filter suggesting they replace it because it looks dirty?!?!

This is really beginning to frustrate me!!  I even googled how to check your car's air filter and the same information is given - "hold up to the light".  Wrong! Wrong! Wrong!
Let's start at the beginning.  I know of two types of air filters for a car - there is a cabin air filter (that cleans the air before going inside the car) and there is the engine air filter.  The engine air filter is the functional one.  Air is required for an engine, no matter what type of fuel it uses - diesel or gasoline.  Clean air is also required, because you definitely don't want to get dirt into your engine!  However, the time to replace an air filter is easily done by following the maintenance schedule in your car's owner's manual.  OR it's done by knowing what restriction value will prevent proper flow to the air intake.  However...that's not easily done for the average joe or jane!
(Interesting piece of knowledge:  An air filter's efficiency - ability to trap dirt - actually increases with use!  However...there is a point where  it does become to clogged that air can not flow.)
Every time I go to get my oil changed, I specifically state that I do not want my air filter touched.  Why?  I've already established that, in my mind, the best way to change the air filter is according to the maintenance schedule - but why does it matter if it's touched??  Because!  Because every time the seal is broken at the air filter housing there is a potential to get dirt into the engine.  (Dirt in engine air supply = bad)  So whenever the current air filter is removed and then reinstalled - there is a high chance that you will get dirt in the engine.
So a summary of what I'm sharing:
1) Looking through an air filter is NOT an accurate way of telling whether the air filter needs to be replaced.
2) Every time the used/current air filter is removed and re-installed the clean-dirty seal is broken and there is a good chance that dirt will get into the engine air supply.  So if you remove the air filter - put in a new one!
So this is my plead:  SAY NO TO AIR FILTER CHECKS during your OIL SERVICE!!
So what do you do?  Simple!  About every year (or more specifically the mileage that your car indicates, depending on if you live in dustier or "cleaner" conditions) go into your local auto parts store and have them help you find the air filter to replace in your car.  It will probably cost you about $10-20.  Then following the instructions in your owners manual or a helpful/knowledgeable person - replace your own air filter!!!

**PLEASE note:  Most cars these days have 2 air filters - one is for the cabin (the air conditioning) and one is for the engine...I'm talking about the one for the engine!