I already sent him a PM on this and the rest of you are idiots. It's simple math.
Divide your resolution and you get your aspect ratio.
1680 divided by 1050 = 1.6
1600 divided by 900 = 1.78
640 divided by 480 = 1.33
1.33, 1.25, 1.36 = 4:3
1.60 = 16:10
1.78 = 16:9
1.85 = 37:20
2.20 = 11:5
2.35 = 47:20
Dividing those aspect ratios give you the same numbers of course. This should help you with the most common resolutions. 4:3 is a weird one because there are several different versions probably due to the varying dimensions of pixels on older CRT screens. I'm not really sure.
This should cover most common aspect ratios. If you have something not listed, try dividing your resolution and doing a google search for that aspect ratio.
Aspect Ratio is just the definition of how wide something is compared to how tall it is. At 37:20 the screen is 1.85 times wider than it is tall. 1.85x20 = 37.
There's a slight discrepancy with the 1.78 aspect ratio because 16 divided by 9 is actually 1.7777777777777777777777777777778 and that number is rounded up. However, 1.7777777777777777777777777777778 x 9 = 16 and the aspect ratio is maintained.
If you have a list of aspect ratios in decimal format, and a list of aspect ratios in w:h format, you can just start dividing the numbers til you get a match for yours. I've done some googling and I can't find an actual chart to back up my post. All I see are aspect ratios listed as decimal or w:h format, but nothing that has both.
You can PM if you can't find it or need help.
Hope that helps, though. It covers the most common.