The supplier should be able to give some guidance (if the asset owner doesn't have any minimum acceptance criteria). Also supplier test results from factory testing may be helpful.
I would expect a new xlpe cable to give a tens of gigaohm reading for substation usage, so would expect something similar here (but I have not done any submarine cable work so am only guessing). If you vlf (very low frequency hipot) the cable with minimal leakage current it would prove the cables is fit for use.
