Join the Theatre Alliance of Greater Philadelphia. The alliance allows all performers in the area to become aware of all theater-oriented news in the area, including auditions. They also host annual auditions where over 50 professional theaters in the Philadelphia area attend a mass audition.
Network with other performers. Inform others of auditions that they would be right for, and often times they will return the favor by giving you information on an audition you may not have found otherwise.
Subscribe to a service that provides "Breakdown services" like Backstage.The breakdown services are postings of productions that are auditioning for specific characters. They will often have a little information about the character so that actors can discern if they are right for a role or not. Backstage has the option to search for opportunities in a specific region, like Philadelphia.