Our genetic code has fixed-length triplet codons, but one can imagine an alien genetic code that uses duplet codons (each 2 nucleotides) or quadruplet codons (each 4 nucleotides).
For example, an RNA sequence ...AGCCGUUCACUAGUCUCAUC... may be read according to a duplet genetic code as:
AGCCG UU CA CU AG UC UC AU C...
or
AGCCGU UC AC UA GU CU CA UC...
According to a quadruplet code, it may be read in any of the following ways:
AGCCG UUCA CUAG UCUC AUC...
AGCCGU UCAC UAGU CUCA UC...
AGCCGUU CACU AGUC UCAU C...
AGCCGUUC ACUA GUCU CAUC...
Note that translation in your in-vitro system (unlike natural translation) can initiate anywhere in the RNA (see IN-VITRO-TRANSLATION).
In the absence of experimental results, any of these possibilities (and many more) may be right. However, you can exclude many of them by assuming them to be true and showing that doing so leads to a contradiction, for example a codon that codes for one amino acid in one context and another in a different context.
