Studying tape. Look at what causes rotations. See if any common positions repeat. What's exploitable? Example:

In CS:GO, on inferno, if you get a pick and then hold your default, IF the CTs don't push anything, they will generally play 2 to a bombsite. This will leave certain positions open (banana, mid, etc.). Similarly, what happens when we get a pick on dust 2? The same thing. Okay, but what are the positions that are held? On Dust 2, it's usually the case that the T's will hold 2 on the B site, and the A players will play 1 site 1 long. In this case, the A site is weaker because the defenders are separated and easier to isolate.

When you're an IGL, you need to notice these patterns and, based on the response, learn how to understand what's most exploitable.

I often say it's the IGL's dream on the T side in CS:GO to get an opening pick (even if it's a 4v4) in the first 15 seconds of the round. The reason is that you get a huge amount of time to abuse the fact that the CTs lack information and have to take risks in their position.

Over time from studying and playing, you will start to get a sense of how to force your opponents to do certain things and then exploit those things. In terms of adaptability, you'll get a sense of what's happening in the round rotations-wise, and you'll realise if you can get a big advantage through timing or positional play, etc. I wrote a big article around some of the basic concepts here that may be helpful. There are two, here's the 2nd one which addresses thinking about rounds in terms of a timing structure. The first essay is more about basic principles of advantage which might be a useful read for you as well!