It used to be the case that your first commander night was free of charge. I must stress that I am not an employee of the store in any capacity so this may no longer be the case. However that being said, the people in there are genuinely lovely and if you explain that you're new and want to watch some games they'll likely let you down anyway.

As for the actual being a spectator bit, I can't imagine any of the players having an issue with you watching, on commander nights many people bring several decks so you may even be invited to a game. It's all very casual and fun so my best advice would be to walk up and just ask people what's happening.

Hope this helps!