A method in a modem having a frame having a plurality of slots, for determining whether incoming DIL sequence information contains A-law or mu-law is provided. From the DIL sequence information, an ordered table of levels for each slot is generated. The levels of all non-robbed-bit slots are averaged, and average levels are subjected to an A-law/mu-law separation function. The separation function F1(n1,n2)=sum from i=n1 to i=n2 {L(i) - 2* [L(i-16]} effectively compares values of a first plurality of levels L(i) to twice the values of a second plurality of levels sixteen levels removed from the first plurality of levels (420). The sum is compared to a threshold (430), as the sum will be small for A-law values and large for mu-law values.