In theme, how can I check if a plugin is enabled?

[Mark Nielsen]

I'm writing a theme which integrates with the I18N Navigation plugin. However, I'd also like the theme to degrade gracefully and not throw errors if I18N Navigation isn't available or is disabled.

Is there a way I can query to see if a module is enabled, so that I run certain code only if it is? Something like:

if ('I18N Navigation' is present and enabled) {
Do some funky stuff that depends on I18N Navigation functions.
} else {
Do some less funky stuff.
}