You get to see an agent "in action" at an open house! While you are there you can gauge their professionalism by the way they greet you, engage you in conversation and talk about the house they are hosting the open house at. Ask questions about the neighborhood, the Seattle real estate market both in general and specifically homes in the neighborhood and comparable homes. While talking to the agent, do you find them knowledgeable, and a good "fit" to work with you?
Check out the agents' marketing materials (flyer's etc.) are they informative and interesting to look at? Ask the agent what they did to help their client get the house "market ready" and their philosophy for marketing and selling homes. Later when you get home, look the house up online to see the quality of the online photos, and the marketing remarks for that house, do you find them to present the home well?
A good agent will be happy to answer all your questions, but be careful not to monopolize their time after-all, they are working to sell their clients home! The agent is there to show their clients' house to potential buyers so they will need to greet and talk with other visitors at the open house too.
Sometimes the agent hosting the open house is not the listing agent, so that is something to find out while talking to the agent. The hosting agent is still a good potential agent for you even if they are not the listing agent. Sometimes a listing agent is not available to host that week's open house for one of several reasons, and another agent is hosting it. Even if the agent you meet and talk to is not the listing agent, that hosting agent may be a good fit for you if they impress you with their professionalism, knowledge and marketing savvy.
Be sure to get the agents' business card and a flyer too so that you can follow up and call them later to talk about what they can do for you to market and sell your home. A little time and effort spent doing this will help you to find the best Seattle agent for you when the time comes to sell your home.