Following filter can be used to filter on fragmented packets.
ip.flags.mf == 1 or ip.frag_offset > 0
When PATH MTU discovery (PMTUD) is enabled, the DONT FRAGMENT bit in the IP header is set and a router that would have to fragment will discard the packet instead and send back an ICMP message indicating what the next hop's MTU size is.
A re-transmission is then required at the sender initiated by the upper layer protocols (TCP,RTP etc) with a then reduced MTU size that will - hopefully - go through unfragmented.
Following filter can be used to find the icmp fragmentation needed packets.
icmp.type==3 and icmp.code==4
You see, combining these filters can result in a lengthy string of expressions.
Here is an abbreviated version that will do the same (for IPv4 packets) and also impress your boss.