Where do I find evidence of what happened to Ambassador Osoba's son?

Solution 1:

You can complete this during the N7 mission Cerberus Abduction, the Mass Effect Wiki has a brief "walkthrough". Note if you're lazy or miss the dog tags (the evidence you need) you can just buy them later on.

From the Wiki:

On the N7: Cerberus Abductions mission, look for dogtags on the ground near a pillar where your first objective is. Return to the Citadel and give the dogtags to Dominic Osoba to finish the quest.

If you miss the dogtags during N7: Cerberus Abductions you can purchase the dogtags from the Spectre operation room for 1000 credits and then present them to Osoba to complete the mission.

Solution 2:

You won't be able to complete this one until you've progressed a bit further in the game from when you first acquire it.

Once you gain the storyline mission Attican Traverse: Krogan Team, the side quest N7: Cerberus Abduction will unlock. When you visit the required planet, you'll find his sons dogtags lying on the ground early on.