Keep in mind that every single school handles of touring differently.
Tours are usually limited in number of people who can sign up. Open houses are, well, open. Some have you register but most you just show up—those can get very overwhelming.
Info can be posted on home pages, calendars, admissions or incoming students sections.
Some schools require sign ups, ask you to print out tickets, have check lists when you sign in, while others don't check at all.
Some tours take place during the school day so you can observe classes. Some have presentations. Others take place during the evening when you might meet students and/or teachers.
Some schools separate students and parents on tours, some keep you with a group, at others you might wander the school on your own.
Some open houses cap at a certain number of attendees.
Every specialized high school hosts an open house except LaGuardia.
Tour/open house info is NOT listed in the high school handbook and there is no repository of that information anywhere.
You'll most likely be visiting a bunch of schools. I put together the tour log below to help you stay organized while touring - you can either print out copies and fill them out, or jot down observations and answers in a notebook. The "three things" at the bottom was a system I set up with my little one that worked well. After every tour Jack would have to tell me the three things that struck him most - could be about anything: architecture, class offerings, uniforms, advisory, the neighborhood, locker availability, etc. Then, when we put his final list together in December those noted memories helped him differentiate between the many schools we saw. I took my own notes and between us we were able to make informed decisions during crunch time.