Declarations can generally be phrased in the following ways:
"Luckily I brought my..."
This is good for Resources Declarations and similar applications. If you have a skill to justify it, the GM may make you roll for the Declaration, or just approve it, depending on how cool, useful, and narratively satisfying it may be. Otherwise it is going to cost a Fate Point.
"Knowing what I know about [topic], I'm pretty sure that..."
This is good for Knowledge Declarations using a variety of skills: Burglary, Scholarship, Contacts, Rapport - endless options. Establishing Aspects about a house you plan to break into; establishing an Aspect about a guard to make bribery more feasible; things like that. Again, if you have a skill to justify it, the GM may make you roll for the Declaration, or just approve it, depending on how cool, useful, and narratively satisfying it may be. Otherwise it is going to cost a Fate Point.
"Luckily there happens to be a [useful item] right over there..."
This is good for Alertness, Investigation, or other similar skills about noticing useful things in one's environment. Establishing a firehose station in a burning building; establishing an easily-accessed janitor's closet for a disguise; establishing a window-washing rig on the roof to facilitate access to an otherwise inaccessible area of a building. Again, if you have a skill to justify it, the GM may make you roll for the Declaration, or just approve it, depending on how cool, useful, and narratively satisfying it may be. Otherwise it is going to cost a Fate Point.
In all of these applications, the GM has a general guideline by which to judge how difficult it is to Declare something, but is also free to just say "yes."