There is is_plugin_active() function.

Other method is to check if functions or classes, specific to required plugin, exist.

For practical example see how it is done in scbFramework.

There is is_plugin_active() function.

Other method is to check if functions or classes, specific to required plugin, exist. Some plugins explicitly define their own constants for this, typically with version number and/or other identifying info.

There has been some developments to make extension dependence logic more generic, such as Plugin Dependecies, but they hadn't gained widespread adoption so far.