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The Wireshark syslog dissector uses known ports - defaults to 514 for syslog over UDP.

rfc3164 and rfc5424 define the message format.
You could look at the PRI field in the header but it's length can vary:

6.2.1.  PRI

   The PRI part MUST have three, four, or five characters and will be
   bound with angle brackets as the first and last characters.  The PRI
   part starts with a leading "<" ('less-than' character, %d60),
   followed by a number, which is followed by a ">" ('greater-than'
   character, %d62).  The number contained within these angle brackets
   is known as the Priority value (PRIVAL) and represents both the
   Facility and Severity.  The Priority value consists of one, two, or
   three decimal integers (ABNF DIGITS) using values of %d48 (for "0")
   through %d57 (for "9").