I have learned that WPS is not secure. There is a WPS button on my XFinity modem; but how do I know if it's enabled so I can know if I need to adjust my router settings (or perhaps decline to use an insecure network that I cannot control)?
If you simply option click on your Wifi icon on the menubar in OSX you will be rewarded with a wealth of information about the connection. - if you see something you don't like then as Lyken says log into your router and change it after,
Once you have determined the Security settings for the network you have joined, you can decide to not use it or connect over VPN or perhaps get the administrator of the modem to change the security.
Also, there is a Wireless Scan tool built into Mountain Lion and it's called Wireless Disagnostics.
You can find it in /System/Library/CoreServices (use the finder's go to folder menu item in the "go menu"). Once you open the program, you can choose the utilities window and run a scan of networks to determine their security even before you join them:
The key thing to know here is that WPS isn't a Wireless protocol to secure a WiFi connection but a protocol to connect a device (e.g. your Mac) to a WiFi router. So instead of selecting the right network and typing the WiFi password into the device you can press a button (either physical or virtual) on both the router and the device and have them figure out the rest themselves.
This feature is fairly secure (because both devices get out of WPS mode after a short time even if no connection has been established), but there is also a PIN based method which each certified WPS product must support. This method can be remotely attacked and broken within hours, giving the attacker access to the WPA/WPA keys and the whole network. That's why it's highly recommended to disable the feature, at least after having set up a network.
In Apple land WPS isn't used very much because neither OS X nor iOS support it. Airport Utiltiy 6.x allows to add WPS printers that way (with either the button or the PIN method) but that's about as far as support goes.
Coming back to your question: To secure your WiFi network against WPS attacks you must disable the feature on your router/modem (if possible at all).