There are some tips to fins a DoS.
If you find a lot of ICMP paquets, and if you llok the decode and find that the packet is fragmented, there is a Ping of dead, some attacks ( smurf, poD ) use ICMP packets, so you will find an excesive amount of packets of this tipe directed to a one or more machines.
When you have a Targa attack, you will find a lot of connections to a single station and in the decode you will find packets of diferent protocols IGMP also, and ICPM packets from one single station to all the stations, since it will report ICPM por Unreacheable. If you look in MAC traffic you could find the origin, since targa send modified packets it uses aleatory IP addresses but the MAC should be in some cases be the same.
Let me create some attacks and I will send you a trace with all this.
My best regards.