Does the case study explain the task and the outcome
A portfolio of final images with no context doesn't show how a designer thinks. A strong case study explains: what the client's task was, what the designer proposed and why, what it actually led to. If it's all "here's a logo, here's a website" with not a word about the task, it's hard to judge whether the decisions were sound.
Are there measurable results in at least some cases
Not every project has numbers — sometimes ongoing brand support just keeps a brand consistent, and that's fine. But if at least one or two cases show something concrete — growth in a metric, volume of work delivered, length of the relationship — that's a sign the designer is working toward a business outcome, not just producing pictures.
Variety versus sameness of style
If every project in the portfolio looks like a variation of the same thing, it may mean the designer applies one template approach to every client instead of starting fresh from each brand's actual task. Reasonable variety across approaches is a good sign.
Long-term relationships with clients
If the portfolio includes clients the designer has worked with for years — not one-off, but ongoing support or a series of consecutive projects — that's a strong indirect signal: the results held up well enough, for long enough, that the client kept coming back instead of looking for someone else.
How they respond to direct questions
A portfolio is only part of the picture. Ask directly: how many revisions are included, what happens if the result isn't liked, are source files handed over. Clear, specific answers to uncomfortable questions say more about a designer than another ten polished screenshots.