That will depends on the technology used. But assuming that you don't want to rely on the state of your HTML object, then I'll recommend you to check the http traffic.
You can do so by monitoring your HTTP traffic, or capturing it using a proxy (e.g. using a HAR capture), and checking that a connection to your video location has been successfully made and with some bytes transmitted.
For Codeception, you can have a look at the module BrowserMob.