Yes, actually there is.
The pancreas, of course, makes insulin. In the final stages of this process, proinsulin is formed. This will be cleaved by an enzyme to form insulin and C-peptide. C-peptide can therefore be found on normal human blood samples.
However, artificially manufactured insulin does not include C-peptide. So if the patient is hypoglycaemic from taking too much (artificial) insulin, the C-peptide level will be low. Aha, you say, but if the cause of his/her hypoglycaemia has nothing primarily to do with insulin levels (e.g. alcoholic binge), then the hypoglycaemia will switch off insulin production. So, the C-peptide level will be low in this case too. Yes, but all is not lost - we need only compare the C-peptide level to the insulin level.
A low insulin level and a low C-peptide level together simply suggest the appropriate insulin response to hypoglycaemia of some other cause. However, if the insulin level is high, but the C-peptide level is low, then the insulin must be artificial.
Incidentally, a fairly rare cause of hypoglycaemia is an insulinoma - an insulin secreting tumour of the pancreas. In this case, both the C-peptide and the insulin levels will be high.