how should i find the correct adapter sequence for use in Cutadapt ?
Cutadapt is an older tool wrapper. Try using
Trimmomatic instead. It already has the standard Illumina adaptors pre-loaded.
If for some reason
Trimmomatic does not find any custom adaptors you may in your data, running
FastQC might give clues about what these are. Or, you might need to backtrack and find out how the library was prepared.