Often, you will want, or need, to know something about the world. The GM should tell you the relevant information that your character would know offhand, or be able to easily observe.

But sometimes, you don't know something. The first thing to do is to ask, GMs are humans and forget to mention things, and other times may assume you know background information that you don't. Sometimes you just forget, and the GM should just tell you.

When that fails, you can gather information. You can technically gather information with any action, because you can use you can try to observe other people and see if they are doing that action or are capable of it, or if a location is condusive to an action.

Flipping it around

All your actions can be used in reverse to gather information.

Let's start with perhaps least useful, the Body actions.

• Grasp would let you check to see how difficult a pickpocketing a specific person is going to be, or use your skill at safecracking to recognize how hard the safe will be to open. It also can allow you to notice if anyone else is pickpocketing, although that basically requires them to do it in front of you.
• Sneak lets you see where you could hide in an area. And, because you know where people could be hiding, you could spot someone hiding there. Although, again, this would require them to be doing it as you looked.
• Assault lets you locate things you could use in a fight, and when used against people, can be used to see if they are carrying concealed weapons or holding themselves as trained combatants, perhaps even scanning the room for threats just like you are.

All of those can be useful, but only in very specific circumstances.

You can also do Modify, Grift, and Order in reverse.

• Modify would be to see if something has been tampered with. This is very useful for hacking, if you are trying to track down what another hacker did.
• Grift would be to try to figure out if someone was lying, although again they'd have to be lying at the time. It also could possibly detect if they are playing a role in some manner, even if not lying, if their outfit is just slightly wrong or their word choice isn't quite correct.
• Order would be figuring out if someone was the legitmate authority, or who the legitimate authority would be. It also can be used to figure out what authority people expect.

The main actions

The remaining actions are what you you would normally use to gather information.

• Locate is the biggest one, the action is almost entirely gathering information, and can be used to do everything above, and try to track down everything else. You just look at the world around you, and notice things. So in that sense, it's a very powerful action, but it does only give you information.
• Connect is basically just asking other people for the information. Sometimes directly, sometimes asking around on the DL, and sometimes tracking them down though a friend of a friend of a friend and getting them to spill secrets over a beer. Remember, you can do this in a flashback if it doesn't fit in the present.
• Understand is the odd one out. You generally can't use it to gather information to start with if you have nothing, you mostly use it when some aspect of the team gathering information fails, and you have gaps in your knowledge. It allows you to make deductions about what is going on anyway. You base these deductions off human nature and a very wide understanding of the world. It's basically the 'genius' archtypical behavior, where you just have 'figured things out' despite not really enough information.

You also can, in theory, use all of these in reverse to determine if someone else is good at that thing, although it could be fairly hard to directly observe any of that.

For an example of how all these work, let's see how Sherlock Holmes would gather information:

Sherlock investigating Sir Reginald Westbridge, so he uses Connect to ask the Baker Street Irregulars when he's been recently. The Irregulars say he's been see frequenting a lot of different bars recently, a widely variety of bars than normally expected, although he doesn't seem to be a drunkard. Sherlock then uses Locate to look at at Reginald, and notices he has a very cheap cufflinks despite otherwise having expensive clothing and being well off, and that he has a specific color of dirt on their shoe. And finally, Sherlock uses Understand to put that information together, realizes that the dirt is from Stepney, a very poor district, which is hardly somewhere that a person like Reginald should be, and realize that Reginald might have pawned his cufflinks, which, when combined with the excessive visits to multiple bars, might indicate a gambling problem he's hiding.

And...that is a lot of leaps of logic. It's not something that a person in real life could do, and in reality there's dozens of reasons that the observed things could be true. Sometimes dirt is just dirt, and his good cufflinks got misplaced and he grabbed a pair without noticing, and he's going to bars because he's had a split with his girlfriend, who turned out to be friends with all his other friends who chose her over him, and he's been barhopping to try to meet new people, while carefully avoiding falling into alcoholism.

However, this is fiction. Sometimes the conclusion may not be entirely true, but the fact it was included in the story means it is relevant, and Sherlock can move forward on the assumptions he's made. Because the story won't be 'dirt is dirt and the cufflinks got misplaced', the story will be that Sherlonk is 90% right, but Reginald is actually covering for his brother's gambling addiction and trying to get him out of those bars. And the narration wanted that original misunderstanding.

Likewise, that's what happens when you use Understanding. Even if you don't get the full picture, you'll get something that the GM wants you to conclude...maybe not true, but it will lead you to true things if you follow the conclusions.