I'm a developmental editor, and I've worked for years with my own editor. Before finding a great editorial match with her, I worked with two other editors. They were very knowledgable, and I appreciated their feedback, but my long-term editor gets my style and humor; she jokes that we must've been separated at birth in terms of our tastes in books/music/film. So, find someone who is a tonal match. Who understands where you want to take your right-now project, but someone who can also support your longterm goals. Many editors do provide sample edits, but I myself have stopped offering these for free; I simply can't afford to do several hours of work with the risk of not getting paid. BUT, if someone does decide to work with me, I deduct the cost of a sample edit from the cost of reviewing the larger work. This is actually pretty common practice. You should also check that your potential editor has a detailed for for hire agreement and a detailed editorial contract (specifying the scope, duration, and timeline for the service for which they're being hired, but also so that you can ensure your rights as a creator are being protected). For any additional questions about what is/isn't common practice, check out the EFA or a similar professional organization for editors.