Honestly people, I think we're beating around the bush and really looking for an absolute fact where strongly implied facts are all we REALLY have to go on.
From my perspective, I've always been under the impression that Riker and Sisko both had served as Chief Engineers at one point and as XOs both on 1-2 ships before their current commands (or in Riker's case, before serving under Jean-Luc).
If you don't agree, then feel free to agree to disagree!
There is one point I feel its worth while tackling: Picard's technical knowledge. Its very obvious Riker's technical know hows very much are heads and tails above Picards. If you need an example of this, just look at the episode Phantasms. Picard is offering thoughts on things to check that are clearly below LaForges "Pay grade". Riker on the other hand, we've seen him really get his hands dirty working in engineering. If anything I would suggest his technical knowledge seems limited on the Enterprise because its so much more advanced then any ship he would have served on in an engineering capacity. Its kind of like having an understand of computers from the Pentium Era and then being asked to work on a computer from the Pentium i7 era. Sure, you can VERY LIKELY figure out what you need to do, but odds are you might need to ask some questions or do some reading.