For the originals, 6cyl coil size is slightly smaller... I dunno on aftermarket, as the 6cyl springs I installed are same size as original V8 springs, they are 1/2 coil shorter though...
The only way is if the color code paint markings are still present. There is a chart to "decode" those. Or, if your really lucky, the factory ID tags sometimes remain. Here is the leaf chart - there is one for the coils too someplace. http://www.1bad6t.com/Maverick/repair/leafspring.html
Shoot....I wish I had seen the coil spring/paint stripe chart earlier when I did mine. I have the build sheet and went off of that. I have lots of build sheets. I will have to check and see how they match up with the chart.