Since the question only specified that Search API quotas should not be used in finding out if the channel is streaming, I thought I would share a sort of work-around method. It might require a bit more work than a simple API call, but it reduces API quota use to practically nothing:
I used a simple Perl GET request to retrieve a Youtube channel’s main page. Several unique elements are found in the HTML of a channel page that is streaming live:
The number of live viewers tag, e.g.
<li>753 watching</li>. TheLIVE NOW
badge tag:<span class="yt-badge yt-badge-live" >Live now</span>.
To ascertain whether a channel is currently streaming live requires a simple match to see if the unique HTML tag is contained in the GET request results. Something like: if ($get_results =~ /$unique_html/) (Perl). Then, an API call can be made only to a channel ID that is actually streaming, in order to obtain the video ID of the stream.
The advantage of this is that you already know the channel is streaming, instead of using thousands of quota points to find out. My test script successfully identifies whether a channel is streaming, by looking in the HTML code for: <span class="yt-badge yt-badge-live" > (note the weird extra spaces in the code from Youtube).
I don’t know what language OP is using, or I would help with a basic GET request in that language. I used Perl, and included browser headers, User Agent and cookies, to look like a normal computer visit.
Youtube’s robots.txt doesn’t seem to forbid crawling a channel’s main page, only the community page of a channel.
Let me know what you think about the pros and cons of this method, and please comment with what might be improved rather than disliking if you find a flaw. Thanks, happy coding!
2020 UPDATE
The yt-badge-live seems to have been deprecated, it no longer reliably shows whether the channel is streaming. Instead, I now check the HTML for this string:
{"text":" watching"}
If I get a match, it means the page is streaming. (Non-streaming channels don’t contain this string.) Again, note the weird extra whitespace. I also escape all the quotation marks since I’m using Perl.