How do I know if my fan is blowing out the exhaust air or blowing in fresh, cold air? And which way is better for a case fan?

well one way is to just feel, and the other is do you see how 1 side of the fan has a plastic bar across it and the other side doesnt? it will blow the air out of the side with the plastic.

you want air coming in from the front and going out the back

Is it the same if the air is coming in from the back and blowing out from the front? Also what if the fan blowing out pushes less air out (~20CFM) than the fan blowing in (~80-100CFM)?

Generally the bulk of the heat is from the CPU, so you want that last in your ventilation trail. If you intake from the rear, you will blow the warm air from the CPU throughout the motherboard.

Generally, you want the same pressure in and out. The warm air will swirl in the top of the case if not.

Also, the HD is usually right in front, so the case fan actually cools the HD(s) pretty good.

cases are made to blow from front to back, so do it that way. you should think about upgrading fans, you need more cfm or else you wont be blowing much air out. or switch around the fans in ur case.

well one way is to just feel, and the other is do you see how 1 side of the fan has a plastic bar across it and the other side doesnt? it will blow the air out of the side with the plastic.

There is one more; almost every fan has arrows on them, one for the “propeller” (rotor) direction and one for the air direction .

Is it the same if the air is coming in from the back and blowing out from the front? Also what if the fan blowing out pushes less air out (~20CFM) than the fan blowing in (~80-100CFM)?

Reversing the air flow isn't good, since the PSU fans always blow outward from the rear. If the air is made to enter from the rear, it'll just exit again out the PSU and "short-circuit" the circulation.

If the output fan blows less than the input, the case will be pressurised.

Is it bad if the case is pressurized?

no its not bad, personaly preferance:

negative pressure, more cfms outward:

+ air will come in though the cracks and ventelate not so ventalated things

- fan efficency decreased

- dust buildup

positive pressure, more cfms inward:

+ air will blow out of the case thus helping the dust problem

- hard to setup

even pressure, eaqual cfms:

+least stress on fans

+/- belived to be the most efficent

Would having no exhaust fan work? That's how my 3-year old computer is like and it's running just fine.

you need one. thats just it, a computer from 3 years ago wont run as hot as computers now.

is there a computer store nearby where u can just buy a fan from? it may not be a good fan but then you will be able to start using ur comp and then u can order a good one later on.

while you CAN run withought a exaust fan it is NOT recomended the preformance increase is so great that it is basicly a necasity today, if the psu has a fan that draws air from the bottem, and a 120mm one at that. then you should be fine if your using a other then stock cooler.

I've made up my mind to get another fan . Should I get the same one? Or does it only need a similar CFM?

simaler cfms is fine, just as long as its not horrably diffrent, 90cfm + 30cfm

more like 50+40cf will be just fine.