Generally there's no indication of protocol in MPLS label

Generally there's no indication of protocol in MPLS label
encoding, however draft-hsmit-isis-aal5mux-00.txt describes
a technique that looks at the first payload byte if the BOS (bottom of stack)
bit is set and tries to determine the network layer protocol
    0x45-0x4f is IPv4
    0x60-0x6f is IPv6
    0x83      is IS-IS
this technique is sometimes known as NULL encapsulation
and decoding is particularly useful for control-plane traffic [BGP]
which cisco by default sends MPLS encapsulated