In short: Vetting an evaluator comes down to specific questions about fit, method, and availability. Credentials alone won't tell you who's right for the work.
Finding an evaluator you can trust is a search most people do once. These pages cover where to look, the questions to ask, the credentials that matter, and how to compare proposals.
A fundable evaluation section names the questions, the design, a defensible budget, and the evaluator, and it's written while the proposal is still being built.
An evaluation that finds a program isn't working is still useful, and there are honest ways to report it that protect the program and the funder relationship at once.
The choice between an outside evaluator and doing it internally turns on independence, capacity, and what the funder requires, and sometimes a full evaluation is more than you need.