Check first two items.
If they are ordered, check next items using recursion:
def isSorted(L):
if len(L) < 2:
return True
return L[0] <= L[1] and isSorted(L[1:])
Side note The function can be expression as a single expression as thefourtheye commented:
return len(L) < 2 or (L[0] <= L[1] and isSorted(L[1:]))