How to parse clientbound and serverbound packets differently?

edit

Can you determine direction programmatically?

One way of doing that is comparing the source and destination port numbers.
Search code for string port >.

epan/dissectors/packet-udp.c:

    /* check direction and get ua lists */
    direction = cmp_address(&pinfo->src, &pinfo->dst);
    /* if the addresses are equal, match the ports instead */
    if (direction == 0) {
        direction = (pinfo->srcport > pinfo->destport) ? 1 : -1;
    }

epan/dissectors/packet-tpm20.c:

    if (pinfo->srcport > pinfo->destport) {
        col_append_sep_str(pinfo->cinfo, COL_INFO, NULL, " [TPM Request]");
        if (length >= TPM_COMMAND_HEADER_LEN)
            dissect_tpm20_tpm_command(tvb, pinfo, tpm_tree);
        else
            dissect_tpm20_platform_command(tvb, pinfo, tpm_tree);

    } else {
        col_append_sep_str(pinfo->cinfo, COL_INFO, NULL, " [TPM Response]");
        if (length >= TPM_COMMAND_HEADER_LEN)
            dissect_tpm20_tpm_response(tvb, pinfo, tpm_tree);
        else
            dissect_tpm20_platform_response(tvb, pinfo, tpm_tree);
    }