Block/Deflect state that "as a reaction, you may negate a melee/ranged attack by making a successful Use the Force check". I'm unclear on the timing of reactions. Do I have to declare block/deflect and roll versus the attack prior to knowing if their attack would have missed regardless? Or may I react with a block/deflect after I know what the attack roll result is?
I could not find anything specific in the rulebook, however on p. 41 in the Jedi class description it states that the difficulty of the Use the Force check is equal to the result of the attack roll you wish to block or deflect. That implies that you make the check after you know the result of the attack.
In other d20 games you sometimes find the phrase: You have to declare you want to use (insert special ability here) after the roll has been made but before the GM has told you the outcome (e. g. reroll a skill check if you think you rolled to low, see p. 59 under Rerolling). However, that is not the case here. As long as the rules don't state that you have to do something I would assume that you don't have to do it.
Also, reactions happen as a reaction to a trigger and take no time (at least not in regard to the action economy in this game in a given round) and it would go against the nature of reactions if you had to declare them beforehand. Then they would not be reactions but instead "anticipations". For example in the skill description section on p. 71 under Perception it states that you can make a Perception check as a reaction whenever you have the chance to notice something and under Pilot it states that you can make a Pilot check as a reaction to avoid a collision. In these cases you don't have to state that you want to take a reaction so I would assume that reactions are handled all the same and happen automatically if you want them to happen in response to a trigger for the specific reaction you want to take. In your case, an enemy attacking you.