Lets say I'm playing Thomas during the throw down at Bianca's party in Grave Peril. I make a Lore roll and tell Harry "Their bellies! Without the blood, they're too weak to fight!" Is that my turn, or can I make another action (at a -1)?
As a declaration, you still take your turn. If it's an assessment it may actually take more than one turn.
Does it make a difference if I use that to place an Aspect, or just give another player a chance to place a Called Shot type of maneuver?
A called shot is probably going to be a declaration though that will change from one group to another. Discovering an existing aspect is an assessment. Placing an aspect is going to be either a declaration or a maneuver - though there's a fine meta-game line between placing and discovering an aspect when using declarations.
Check out YS113 - Creating and Discovering Aspects. You can guess by spending fate to use it, make an assessment, make a declaration, or perform a maneuver. As for time taken,
"All assessment efforts require the use of a significant chunk of time..." (YS115) suggests most assessments will take at least one exchange and probably more. I would suggest that time requirement doesn't apply to guessing. On the other hand, declarations are free actions* by default (YS116) and maneuvers normally take an exchange.
*I tend to put soft limits on declarations. It's also worth noting that enough free actions (some arbitrary number) do become a supplemental action.
Generally if you have to roll dice or spend a fate point instead of rolling dice, it counts as an action.
Not always - specifically not when making declarations. I'd also suggest guessing an aspect shouldn't require an action.