When deciding whether to respond, look for obvious signals that you don't need to. If it's a group email sharing some information or an article that might be interesting, it's certainly not necessary for everyone to respond. Many group emails don't merit a reply. Exceptions would be if they clearly ask for an RSVP or an acknowledgement. If the email is sharing information about a conference a long time in the future that you don't need to sign up for, you probably don't need to reply. If it's about a team lunch or a request for acknowledgement that you've read the new procedures, then do respond.
Check for questions in any email you receive. If there's a question to respond to, make an effort to reply quickly and by the end of the day. Don't put off responding to questions if you know the answer right away. If you need to do some research or ask someone else, a bit more time is acceptable, but if it's going to take more than a few days, let the sender know. If you find that you have questions or need clarification about anything, it's also important to reply and clear things up promptly.
When there's not a question involved, but you feel you can add something to a group discussion, feel free to share your opinion. However, consider whether you really need to “reply all” or if you should privately address the sender. If it's not relevant to everyone, don't send it to everyone.
Try to follow the golden rule by imagining yourself in the position of the sender. Would you appreciate a reply or would you be annoyed to get another unnecessary message in your inbox?
Look for obvious cues
Respond quickly if there's a question
Reply if you can add something of value
Put yourself in the sender's shoes