Yes, it's possible to differentiate between these two use cases when looking at network traffic. The simple explanation is:
- When you're downloading the raw video file with
youtube-dl, you're loading a complete file at once.
- When you're watching YouTube video through the browser, the Flash client downloads the video in chunks. The chunks fill up a buffer, and once that buffer is about to run out, the player fetches the next chunks.
Both can be done through HTTP these days. You can observe the client behavior when you load up a video. It is never completely downloaded at once: The buffer will be played out, then the next part will be loaded. This of course is visible in network traffic, as multiple requests are sent to YouTube for one resource over the course of time.
To cite Kuschnig et al. (see below):
segment is split into chunks of size lch, which are served
by a standard HTTP server. The download of the video
chunks is coordinated by the client. For that purpose, the
client maintains nc HTTP-based request-response streams
and schedules the downloads of the diﬀerent chunks by using a separate queue for each stream
If you want more specifics about the YouTube streaming traffic, I could of course explain more. We currently conduct various simulated experiments regarding optimization of YouTube buffering and analysis of diverse video streaming scenarios.
Kuschnig, Robert, Ingo Kofler, and Hermann Hellwagner. "Evaluation of http-based request-response streams for internet video streaming." Proceedings of the second annual ACM conference on Multimedia systems. ACM, 2011 (PDF)
Stockhammer, Thomas. "Dynamic adaptive streaming over HTTP--: standards and design principles." Proceedings of the second annual ACM conference on Multimedia systems. ACM, 2011. (PDF)