Gizmodo has answers!
"The law in 38 states plainly allows citizens to record police, as long as you don't physically interfere with their work. Police might still unfairly harass you, detain you, or confiscate your camera. They might even arrest you for some catchall misdemeanor such as obstruction of justice or disorderly conduct. But you will not be charged for illegally recording police.
Twelve states-California, Connecticut, Florida, Illinois, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, Pennsylvania, and Washington-require the consent of all parties for you to record a conversation.
However, all but 2 of these states-Massachusetts and Illinois-have an "expectation of privacy provision" to their all-party laws that courts have ruled does not apply to on-duty police (or anyone in public). In other words, it's technically legal in those 48 states to openly record on-duty police."