Verify licensing before anything else, through the Commission's public register, since that is the only check that is both free and actually meaningful. From there, ask for the agent's own sale-to-list ratio and average days on market over the last 12 months, not a citywide or brokerage-wide figure that tells you nothing about them specifically.
Seller closing costs generally run 4 to 6 percent of the sale price regardless of which agent you choose, so the number that should actually change your decision is the listing price itself and the reasoning behind it, not the commission structure.
Joel's hand-reviewed valuation lays out that reasoning, including the improvements to your house that public records never captured. Get your hand-reviewed valuation from Joel Dyck.