Easiest way to tell a C40-8W from a C44-9W is by looking at the trucks. The Dash-8s have the older style truck like every 6-axle GE since the U30C, while the Dash-9s have the new Hi-Ad truck nicknamed by fans as “Rollerblade”. The AC4400CWs will either have Hi-Ad trucks or GE’s self-steering truck.
There is no visible difference between a C40-9W and a C44-9W, Norfolk Southern had GE write the control software to limit maximum horsepower to 4000hp. to save fuel and repair costs. Change the software in the locomotive’s computer and a C40-9W becomes a C44-9W. Norfolk Southern does the same with the newer GEVO locomotives.
As others have stated, the trucks are a dead giveaway between -8 and -9. The -8 radiator housing is also ‘thinner’ than the -9 housing. Also, the shape of the fuel tanks tends to give one away - the -8 has the angular fuel tank of the earlier -7s and U-boats. The -9 tank looks a little bit like a cross between that and an EMD tank. The new EMD tanks such as on the ACe and M-2 look similar to the -9 tanks.
To distinguish a -9 from an AC4400 is to look at the location of the dynamic brake vents. These are the square, louvered vents, 2 per side, that are on the ‘hump’ of the long hood behind the cab. On the -9, these vents are close to the cab while on the AC, they’re closer to the engine air intakes on the rear portion of the ‘hump’. Telling the AC and DC GEVOs apart is a different matter.
The difference between various GE locomotives is much more subtle than with some EMD units. From 1/4 mile away though brush, your best bet is binoculars. If you don’t have those, then you’re not likely to know short of recording the unit number (if you can see it) and comparing it with that railroads’ roster.
Ain’t that the truth! Once upon a time you could easily tell an EMD from a GE from a ALCO. Not anymore! There are no more classy chassis, they all are ugly.