You can try the following:
Create some word long variables eg
B0 var WORD
B1 var WORD
B2 var WORD
B3 var WORD

SerIn porta.3, N1200,,, B0, B1, B2, B3
then compare the variables if it contain the same data then it will be the correct code because the encoder/decoder chips send out a 12 bit data that will fit into a word variable that can take up to 16 bits of data