Reliability is not a property of an agency but of a team, and teams change. In the Belgian agency market, median staff turnover was 34.6 percent of headcount in financial year 2024; at a quarter of agencies more than two in three walked out the door.
That figure changes the question. Not "is this agency reliable", but "who stays, and what happens if that person leaves".
What you can check
Financial continuity. Four of the five health figures are free in the filed annual accounts, and the factsheets work them out per agency next to the market median. They say nothing about quality, but they do say whether the agency will still be there in a year. See how to know whether an agency is financially healthy.
Legal continuity. The Crossroads Bank and the Official Gazette show director changes, name changes and mergers. Three director changes in two years is a conversation topic.
Reputation with clients the agency did not put forward itself. Call two and ask about the moment it went wrong.
What to ask about the people
- How long have the people on my account been here?
- What happens if the most important of them leaves: do I get a replacement, and who decides who that is?
- How many people left the team I would get over the past year?
The third question sounds indiscreet and is not: you are buying a team for years, and turnover in that team is part of what you are buying.
Write down what matters to you
If the presence of particular people was decisive, put their names in the contract, with an agreement on what happens if they are replaced. That is not distrust but a normal clause, and it makes the promise from the pitch room checkable.
See also: how to know whether an agency is good and who works at an agency.