Is there a way in wireshark to find out a single SYN that does not get answered by SYN/ACK?
According to https://osqa-ask.wireshark.org/questions/60995/how-do-i-filter-tcp-connection-with-syn-and-syn-ack-without-ack-response/ you can do this by applying a filter
(tcp.flags==0x12) and not tcp.analysis.initial_rtt. The first part finds SYN packets, and the second part find packets for which the return trip time analysis hasn't happened - which implies that Wireshark hasn't seen a corresponding ACK.
Obviously, this will also find SYN packets which were ACKed after you stopped the capture, so use a long capture.
Edit This filter was too narrow, it should be:
tcp.flags.syn==1 and not tcp.analysis.initial_rtt