In my area, there are many Buddhists centers referring to themselves as centers, meditation centers, or monasteries. These all seem to be various expressions of a Buddhist Sangha. However, being in the Mid-West one tends to expect the Sangha to be modeled on or similar to a church or synagogue. What are the proper expectations to have about being part of a Sangha? Should Westerns expect a sangha to conform to the Western notion of religious institutions?
It depends on which Buddhist tradition that particular temple belongs to.
To avoid rampant anti-Japanese racism in America stemming from the 1940s, when Japanese Americans were placed in internment camps, Japanese Buddhist branches like Jodo Shu and Jodo Shinshu conformed themselves to a more western model of creating religious spaces when they settled in America. This is why the American branch of Jodo Shinshu is called "Buddhist Churches of America". They often have pews, a minister speaking from the pulpit, youth groups, etc.
Temples from other traditions usually don't model their space after western religious models. The sangha's "mode of operation" depends on the tradition, which school of thought that sangha follows within it's own tradition, and of course, the Buddhists themselves. So when visiting, these sanghas should be understood on their own terms as much as possible, leaving expectations at the door.
The proper expectations are to have are no expectations.
Speaking mostly from the Theravada perspective I will say there will often be a large difference between a meditation center and a Vihara(monastery). A monastery can be a meditation center, but not all meditation centers are monasteries.
Visit the website and call ahead to see if they have any events open to the public. There may not be regular public "Sunday service" type activities and showing up out of the blue might not be the best depending on the size of the place. Visit when you are able, meet people, learn about how they practice, and join along if you feel it's right for you. You may experience some culture shock, you may not, it is a learning process and the least amount of expectations you have the more you are open to the experience.