The first metric I look at after posting a reel is the retention graph, which is how people behave in the first few seconds of the video, I don't pay attention to likes or initial comments because those are vanity metrics that don't tell you anything useful at the beginning, what really matters is whether people stay to watch the video or leave before it ends, instagram shows you a retention graph in the statistics, and what I look for is that the curve doesn't drop sharply in the first two or three seconds.
If I see that retention is good, that people stay until the end, then I start paying attention to other metrics like direct message shares, which are currently the strongest signal for the algorithm, if people share your reel with a friend, it's because they were genuinely interested, and the algorithm interprets that as valuable content, I also look at the average watch time because a reel that people watch completely has a much better chance of reaching new audiences.
Likes and comments are a gauge of your audience's engagement, but retention is the engine that drives the algorithm to show your content to more people, a reel can have few likes and still perform well if retention is high, that's why the first metric I always check is the retention curve, because it tells you if you're truly engaging people from the very first second, if the curve drops sharply, something went wrong in the first few seconds, and you need to learn from that for the next reel.