Wireshark LUA dissector

Hello all

I am a newbie in Wireshark LUA, and are trying to make a dissector, to analyse a special protocol we use at my work.
I am trying to determine, how to know if a certain packet is for me, and it could be either defines by port (2 valid options) or via a certain value in the data payload (the characters “S” and “C”, decimal 83 and 67).
But how do I test for one of 2 ports in my dissector, and how do I test inside data payload, before knowing it is the right protocol??