Checklist
Feature description
Just as videos you've watched show the watchtime in a red bar on the bottom, I want videos you see in the wild (e.g. suggested under a video you're watching, or in trending) to show something to mark them as in a playlist you've made (to my mind marking videos from playlists you've saved would be less helpful unless marked differently).
Similar idea for channels you've subscribed to too.
Why do you want this feature?
For Playlists:
Not accidentally putting a video in a "watch later" style playlist twice or in two different playlists.
Reminding yourself to get back to a video you put in a "to get back to" style playlist in case you accidentally don't see it.
For Channel Subscribtions:
Not accidentally watching a video from a different channel with a similar name or style of titling + thumbnailing.
Generally (in my opinion) showing more information with little to no extra eye clutter is always a good thing.
Additional information
I made a sketch on a screenshot of some examples that might work:

Additionally, in case anything I added is too small to see, I made one with large red circles around them to mark them out (messy, I know):

Ideas in these examples (playlists):
- Underlining video's name on videos in a playlist
- Putting a mark next to video's name of videos in a playlist (either side)
- Writing Saved or Playlisted next to video age
- Putting a marking next to video age or channel name
- Putting a marking on the top corner of a video thumbnail
- Putting a box around the whole video (thinner then shown)
- Putting a box around video the thumbnail
Ideas in these examples (subscribed channels)
- Putting the channel name in white
- Underlining the channel name
- Putting a marking next to the channel name (either side)
Checklist
Feature description
Just as videos you've watched show the watchtime in a red bar on the bottom, I want videos you see in the wild (e.g. suggested under a video you're watching, or in trending) to show something to mark them as in a playlist you've made (to my mind marking videos from playlists you've saved would be less helpful unless marked differently).
Similar idea for channels you've subscribed to too.
Why do you want this feature?
For Playlists:
Not accidentally putting a video in a "watch later" style playlist twice or in two different playlists.
Reminding yourself to get back to a video you put in a "to get back to" style playlist in case you accidentally don't see it.
For Channel Subscribtions:
Not accidentally watching a video from a different channel with a similar name or style of titling + thumbnailing.
Generally (in my opinion) showing more information with little to no extra eye clutter is always a good thing.
Additional information
I made a sketch on a screenshot of some examples that might work:
Additionally, in case anything I added is too small to see, I made one with large red circles around them to mark them out (messy, I know):
Ideas in these examples (playlists):
Ideas in these examples (subscribed channels)