The plugin's documentation should tell you what you need to know about how it changes the DOM. If it doesn't, it's poorly-documented.
If the plugin is poorly-documented, the basic approach (if you want to use it) is to apply the plugin to your page and use the tools built into your browser to see how it changes the DOM. All modern browsers have a set of quite useful tools built into them; look through the menus (or press F12, which is emerging as the standard keystroke for opening them).
Once you know how the plugin changes the DOM, you can write selectors to target the changes. Be sure you don't try to use those selectors until after the plugin has applied its changes, as they won't be relevant yet. So either don't use them until the plugin has done its thing, or use them in delegated handlers.