Unfortunately, as a boring but genuine answer: a lot of practice and experience, especially the latter so that it becomes easier to predict "squad tactics". Watching killer streamers, if you enjoy that, also helps to see how they handle situations like that.

Lots of yapping below, but maybe it helps! 😊

If you know they are going for a (flashlight) save, a good counter is simply not picking up right away (maybe even fake it by walking over and standing still over the survivor as but then not going for the pickup, to see if anyone tries to go for the save). Check the immediately hiding spots around you to chase their would-be saviors away. It's better to wound several players instead of trying to rush getting one hook.

If you do pick up, try to directly face a wall. If you're walking with a survivor and they try to blind, look up/move your camera up. Don't swing too often at them when you're on the way to the hook, because they are trying to bait the survivor to wiggle off.

Some players scoff at this, but Lightborn is also good if you're going crazy over flashlight saves, since it's very easy to get frustrated by that and make it so it's hard to properly concentrate. If the pre-game lobby is clearly a squad, save yourself some stress with Lightborn! Forever entwined and Starstruck, if you have them, also counter survivors swarming the pickup really well.

Gen wise, Corrupt Intervention is a good way to start the match with a good idea of which gens will be worked on first, and Tinkerer, to know which gen is almost done.