Which pins sense collisions at 100BaseT?
Half-duplex base-T Ethernet does not mean that the same wires are used for both directions of transmission, only that the two ends cannot transmit simultaneously.

Full-duplex means that both ends can transmit and receive at the same time, which only became possible with the shift from the shared coaxial cable to point-to-point unshielded twisted pair cabling.

There are never any physical collisions with base-T Ethernet, only "logical" collisions within a hub or switch. Even with Gigabit Ethernet, in which all four pairs are used in both directions simultaneously, each end uses a "digital hybrid" to separate incoming data from outgoing data on each pair.