If you are not sure if you have viewed a video on YouTube earlier, then you you can use one of their features which puts a tag once you view a video. This is helpful if you are searching for a particular video and there are a lot of videos with same or similar tags. When you watch a video, a small tag “previously viewed” gets added next to the video as shown in the image below :
How Will I Know If I Have Previously Viewed A Video On YouTube ?

You can notice in the image that the first and the third videos are viewed but not the second one. This helps you track all the videos you are viewed and hence saves time while u are searching for a particular video.

To use this feature, you need to be logged into your YouTube account (related reading: recover deleted youtube account).

Recommended reading: Convert YouTube Songs To MP3.