I feel the need to challenge this. I am a former linesman, and have officiated at Maine Road and Old Trafford. It is quite easy and commonplace to judge offside following a pass from another player who is in the other half of the pitch when he makes the pass.
For one thing, in this situation, there is unlikely to be a high degree of opposition pressure on the passer, if he is making a 50 yard pass. It is either a free kick, or he is in plenty of space. So, as he draws back his leg to make the kick, you can very easily compute the moment of contact with the ball, and glance along the line of the defence (where you should be positioned) and capture that picture in your mind at the moment the pass is made. You can also often hear when the kicker's foot impacts the ball, and judge the offside at the exact time you hear that sound. Experienced linesmen and lineswomen are able to make those close judgement calls and should not be making regular mistakes on offside decisions. If Aguero was given offside incorrectly on more than one occasion in quick succession, then the linesman is not doing his job properly. He is either bad at his job, or a cheat.
Another point which I sometimes think could have a bearing on decision making from officials is which team they truly support. We hear stories of referees like Mason and Taylor supporting United, but declaring allegiance to other teams (Bolton and Altrincham in these cases). Now Taylor is from Wythenshawe, as I am originally. I can honestly say I do not know one person from Wythenshawe whose favourite club is Altrincham. It is either City or United. And I grew up in a time when Altrincham were quite a successful side in cup competitions, with quite a few giant-killing achievements.
When this current batch of referees were young, maybe 20-30 years ago, who did they support? The chances are they supported either their home town team, or barring that, the most successful team of the time. That would be Manchester United or Liverpool in most cases. I suspect many referees were United or Liverpool fans as children, and those allegiances (as well as certain prejudices) are now deeply embedded in their psyche.