17 out of the top 20 witnesses have to run the updated code for a hard fork to "activate". You can see who the top 20 witnesses are currently at this link: https://steemd.com/witnesses

When it comes close to the time of the hard fork, you can see which witnesses are running the updated code by looking at the "version" column in the witnesses page linked above, but by that time the decision will already have been made and even witnesses who may not have supported the change will likely be running the new code too, otherwise they will no longer be part of the main chain once the HF happens.

So long story short, the community relies on the witnesses talking or posting about their positions on these matters, which is why it's important for witnesses to communicate.