A portfolio is useful for understanding a team's capabilities, but it should not be the only part of your evaluation. The way a team handles unclear requirements can reveal much more about how they work.
Give a potential partner a real problem from your product and see what questions they ask. Strong teams investigate assumptions, identify risks, and clarify requirements before rushing toward a proposal.
Ask who will actually work on the project after the sales process. Senior people leading a discussion is different from senior engineers being directly involved throughout delivery.
Finally, pay attention to how the team discusses trade-offs. A strong engineering partner should be willing to recommend a simpler technical approach when it delivers the same business value.