In the last year or so I’ve become aware of this meaning for the word “agency”
the capacity, condition, or state of acting or of exerting power
In a recent episode of a popular television show, there was dialog along these lines…
“He wasn’t questioning if he had agency. He was wondering which choice to make.”
I suspect this meaning is actually older than the “office of an agent” meaning but has been out of use for sometime. I’m an old guy who reads a lot and this usage is reletively new to me.
So I’m wondering how do you research this particular meaning of a word mostly, nowadays, used for another meaning?
Is this really a newly popularized meaning, or am I just out of the loop? How do I find out?
Get access to a good etymological dictionary like the Oxford English Dictionary which will provide information about the various meanings of particular words and when those meanings were used.
P.S. I don’t have a subscription to the OED on this computer, but Googlengrams suggests that this meaning of “agency” took off in 1990 or so Google Ngram Viewer
P.S. Actually a search for “having agency” finds a bunch of examples that don’t fit what you’re looking for. However, I did find a 2004 example sentence " By having agency I mean having the ability to make decisions and act in accord with those decisions," (https://www.tandfonline.com/doi/pdf/10.1080/13596740400200187) which is the usage you’re looking for - but note that the author had to explain what he meant, so the usage was probably pretty new - in 2004.
For example, looking up agency tells me that the “exerting power” usage is from the 1600s, whereas an “establishment where business is done” is first recorded in 1861.
Ability or capacity to act or exert power; active working or operation; action, activity.In quot. 1606 as a count noun: an action, a force.
1606 E. Forset Compar. Disc. Bodies Nat. & Politique 86 Bold busie bodies that…thrust in;…their agencies being too daungerous, where [etc.].
1658 H. Slingsby Father’s Legacy in Diary (1836) 208 Privacy…if your Hours in it are not well employed, may become as dangerous as a place of agency.
1697 J. Sergeant Solid Philos. 217 Which…has the truest Notion of Agency in it, without any Mixture of Patiency; because the Body moved cannot re-act upon it.
1754 J. Edwards Careful Enq. Freedom of Will i. v. 30 The moral Agency of the Supreme Being…differs in that Respect from the moral Agency of created intelligent Beings.
1830 S. T. Coleridge On Constit. Church & State 140 The State shall leave the largest portion of personal free agency to each of its citizens, that is compatible with the free agency of all.
1911 Pop. Sci. Monthly May 463 At certain stages of organic evolution consciousness appears as a kind of byproduct and has no agency in the life drama itself.
1994 L. Cochran & J. Laub Becoming Agent vi. 176 A model is…capable of inspiring a person, providing strong motivation to strive for more agency.
2005 R. Toor Pig & I (2009) viii. 179 This…fantasy of giving up agency, of being stripped of having to make hard choices.
Getting back to the OP, the how of how you research usage is very difficult.
The OED solved that problem by creating a whole corps of volunteer readers. They would read through old printed material - books, journals, newspapers, government documents - and write down the complete sentence in which a usage occurred, along with the bibliographic information of the source. They sent these in to the OED, where editors would place these on “slips” - the equivalent of index cards - and file them by word. When an editor got to that part of the alphabet, he - always he in the 19th century - would go through all the slips and sort them into usages and try to compose a pithy definition based on those real-world examples. This, of course, worked better at some times than others. The usage would be backed by the most authoritative original source, so the OED disproportionately quotes people like Samuel Coleridge, although they had little choice when it came to first usages.
Eventually, the slips grew into the millions and that allowed them the luxury to trace usages over time and see when new ones appeared or old ones become obsolete. It’s still done the same way, though computers make everything easier. That’s why it’s almost impossible for any individual to trace usage, even with Google Books and newspaper databases. No single person can read that much and that widely. Hundreds, thousands, of people are needed.