Those strings don't appear in the packets; they come from Wireshark, which interprets the numerical value of the SMB request code.
But if all you want is to detect SMB1, and the auditing Graham Bloice mentions isn't possible, that's more than you need; you don't need to look for particular SMB messages, you just need to look for SMB1 messages of any type.
You could try looking for any TCP packets to or from port 139 or port 445 in which the first byte of the TCP payload is 0 (a NetBIOS-over-TCP "session message", or a regular SMB-over-TCP message) and bytes 5, 6, 7 and 8 are 0xff 0x5e 0x4d 0x42:
(tcp port 139 or 445) and tcp[((tcp[12:1] & 0xF0) >> 2):1] = 0x00 and tcp[((tcp[12:1] & 0xF0) >> 2) + 4:4] = 0xff534d42
(The filter fetches the data offset from the TCP header, multiplies it by 4, and adds it to the 20 and 24 in the TCP payload test, so that it works even with TCP segments that have TCP options.)
(This reminds me - I really need to add string comparison, byte-string comparison, and a direct way to access the UDP and TCP payload to the capture filter mechanism in libpcap....)