Basically you can't know, because the manufacturers made up their own systems. General Electric designated wide noses with a W and Electro Motive Diesel/Division (Depending on who owns them, currently it's Diesel) designated them with an M, just because that's what they decided they wanted to do. American locomotive manufacturers basically each pull their very own numbering systems out of their asses, and they change them all the time too.
The 70 in SD70 is basically an incremented version number. EMD's first two Special Duty locomotives coincided with the last F units, the F7 and F9, so they kept the same numbers for consistent branding. They stopped making Fs so for the next SD they doubled the last number to get 18, then they added 6 to get to the SD24 for no apparent reason. The next major model was the SD35 which matched the GP35 (As 4 axle locomotives were much more popular during this time) and then the SD38 to match the GP38. Finally with the beginning of the 40-50-60 series EMD settled on a somewhat standardized system, so we saw the GP40/SD40, (Very popular) the GP50/SD50 (Not very popular, poor efficiency and reliability) and then the GP60/SD60 which were met with reasonable success, although the SD60 much more than the GP60 as 4 axle locomotives were falling out of favor. The SD60 spawned a variety of variations, most notably the SD60M and SD60MAC. The next model was the SD70, which was interesting in that there wasn't a corresponding GP70 since there just wasn't a market for one. Just like with the SD60, there was the widenose SD70M and the AC SD70MAC. Then came the 5000 horsepower SD80MAC and the 6000 horsepower SD90MAC but neither were very successful since more 4000-4500 hp locomotives gives railroads more flexibility than fewer 5000 or 6000 hp locomotives.
I guess someone decided they couldn't make the SD100 since it would be less powerful than the SD90, so they went back to the 4000 hp SD70. The DC version is the SD70M-2, originally the -2 was for versions of the 40-50-60 series with upgraded electronics, but they brought it back to differentiate it from the original SD70M. The SD70ACe is the AC equivalent to the SD70M-2, I guess the 'e' stands for enhanced or something.
TL;DR In EMD's numbering scheme the 70 is a version number not the horsepower.
Also, only a handful of F9s were built as dual-mode units, the vast majority were standard diesel-electrics. The ones that were dual mode are called the FL9 since the 'DM' is a GE thing.