I think most nurses have a pretty good sense of when other nurses are drowning. Whenever I'm super busy dealing with a decompensation or admission nearby nurses are typically either helping me or if they are too busy themselves, they have the courtesy not to ask me for help until I've got what I'm dealing with sorted.
Whereas I often have little stretches between my tasks where I will float around and see if anyone needs help. These are the best times to offer help because if you offering help when you really are quite busy the help you are offering won't be overly helpful (wow that was a lot of helps in one sentence lol).
I think frequent communication with colleagues is important to teamwork. I'm often working in a specific clinical area with 1-3 other nurses and I communicate when I have important things going on so we can coordinate tasks that require 2 nurses as well as breaks.
When you are offering help I think the most important thing to do is being specific in your offer. Particularly when someone is being held up with one busy patient. "Do you have any meds so for your other patients that I can do?" "Do you want me to do a round on your other patients?" "Should I call the on call doctor to get some orders for you?" Ect. Offer things that you can reasonably do within your current workload. Sometimes people ask me if I need anything, but they don't mean anything. They don't have time to stay for a 20 min procedure. So communication is key.