For some reason, this question was not readily answered anywhere. I thought I would find them in the “active_plugins” key in wp_options (whereas the per-site plugins are in wp_XX_options). But it turns out wp_options contains the active plugins (and other data) for the primary site in the network.
I turned to the Heroes at WPMU DEV for help, and Adam helped me out:
Look for “active_sitewide_plugins” key inside the “wp_sitemeta” table 🙂 You’ll find network active plugins there 🙂
Hooray for Heroes!