At the time of this post, I believe all ECM is located on the left torso on capable mechs (so, on the right side when you're facing them head-on). The exception is the Kit-Fox, which has a non-standard omnipod left arm you can get for it.
For the Atlas-DDC, that'll be the missile side that does not have the big a*s gun(s). Usually people will look to knock out the AC/20 on the DDC first though.
For the Spider and Commando variants that can mount ECM, that info probably isn't really helpful because most folks can't focus that precise torso shots on fast-moving lights anyway. The good thing is, those models have a lower engine cap than the others, so untweaked, the fastest they go is about 135 kph (~150 kph tweaked). This max speed is also about what the Raven-3L can get. Interestingly enough, the Cicada-3M has a similar max speed. However, a lot of people don't use the biggest engine in that mech so they can mount more weapons. People almost always use XL engines in these mechs so if you manage to completely destroy that left torso, that's a plus too.
The Kit-Fox is a slow light which makes it a little easier to take out that left arm. That arm is actually also capable of mounting three AMS, so if you see a Kit Fox with some pill thingies sticking out of its arm, it's likely got ECM in that arm as well.
For Clan Wave 2, I believe there are 2 mechs that have ECM-capable omnipods: The Mist Lynx (25 tonner, left arm) and the Hellbringer (65 tonner, left torso). I dunno if there will be any additional omnipods like the Kit-Fox has that can mount ECM.
Edited by Takashi Uchida, 14 October 2014 - 04:53 PM.