The client list is the first thing you see on an agency site and the least informative thing on it. What you want to know sits in two questions around it.
Where the list is
IKAg's factsheets carry the reference clients an agency states itself, along with the sectors it works in. You can also search the other way round: by sector you see which agencies are active in your market.
Beyond that, the trade press reports who won which assignment, and award sites show the client and the agency per campaign. Those two sources are more useful than a wall of logos, because they carry a date.
The two questions that pay off
"Which client left in the past year, and why?" Every agency portfolio loses clients; that is normal. What you want to hear is whether they name it and what they learned. An agency saying nobody left has either a short list or a short memory.
"Which client sits in my category?" That is not curiosity but a condition. An agency serving your direct competitor usually cannot take your assignment, and if it wants to anyway, you need to know how separation is arranged: separate teams, separate files, and an agreement on who sees what.
What a logo on a site does not say
- Whether the work is still running. Logos stay up for years.
- How large the assignment was. A three-week project produces the same logo as a seven-year relationship.
- Who did it. The team that served that client may be working elsewhere now.
So dig into two or three names: since when, what exactly, and who from that team is still there.
A detail you can use in your favour
An agency with a long list from your sector knows your market, but has also sold the same solution there five times. An agency without that list has to learn, and sometimes arrives with something your competitors did not think of. Both are defensible; the choice depends on what you are after.
See also: how to know whether an agency has experience in your sector and what belongs in an agency contract.