This is something you need not worry about if are just going to watch your movies with your family. It's the people that upload these to torrent sites that need to worry. The watermarks are invisible, if they arent I have never seen them.
Not uploading torrents does not necessarily mean you're safe. If you'd put your files on NAS and decide to share it over DLNA to your TV, everyone who you share your Wi-Fi with is able to download the data. Of course it's rather marginal example, it would mean many things, including either you wrongly choose people around you or some people are jerks. But still, we're giving our Wi-Fi passwords to our friends and we don't ask what stuff they've got on their devices and if they take care of updates (so they're not unconsciously checking all of your devices).
Back to the topic - the only way to check it, IMO, is a kind of reverse-engineering attempt by downloading an item several times using different accounts, maybe from different IPs (but from the same region and using the same method), extract tracks and compare them. I don't think that providers have enough power to encode a video stream for every user yet, as it is going with watermarked audiobooks. I don't know, however, what kind of things to check, as everything may be a part of steganography. in HLS or DASH streams, for example, we interact with playlists which contain chunks of video/audio. If the provider has a given quality stream encoded as two slightly different but interchangeable copies (e.g. P+B+I frames at the same time, the same encoder settings but unnoticeable bitrate/quality difference...), the server-side orchestration software is able to serve different chunks' combination for each user. And there may be more fuses than this one.
Sceners, I think, have either a possibility to obscure the mechanism by mixing two or more streams together, doing other things they think it's necessary or not caring at all, as they usually can use bogus accounts. That's my idea, I don't know how it works, and I suppose I don't know way many more thinks than I know. Hence, I like the ability to download the content uselessly and use it on whichever capable device I can (I don't like DRMs, I like fair customer/provider approach), but I would recommend avoiding sharing it somewhere else (and the possibility of being "catched" is closer to the end of the list).
