Didn't harry do a soul-gaze on some guy to find out he was being manipulated by a WCV? Soul gaze is a bit more personal than the Sight, though.
The Sight shows a person's aura (or that's how I see it). Many things can affect a person's aura: magic, illusions, emotions, addiction, affliction etc...
All the swirling colours and images are there and they represent all those things. Some of them are very subtle and some of them are obvious.
It might be hard to tell someone is a bit depressed because their cat died last month but much easier to tell they have a huge, powerful curse on them.
On the same token, It might be hard to noticed faint magic but easy to see that someone is so depressed they are suicidal.
That's what the Lore check is for. It gives aspects and information. The more shifts they get, the more information they should receive.
Beware that they don't use the Sight all the time. I think the base difficulty recommended in the book for trying to close the Sight (and interpret it is 4 or 5). Otherwise, it becomes 'detect magic/True Seeing' for DFRPG.
I don't think Harry likes opening the Sight. Because it's freaky and dangerous.