As the victim (the one being shot), just be as conscious as you can to whether you've been shot. If you think you were hit, call it.
As the shooter, if a guy doesn't call a hit shoot again, if he still doesn't call it aim for a soft spot or full auto. If he still doesn't call it that's when I take myself out, walk over to verify who it is not calling hits and go talk to a game host/admin. Confronting someone in the heat of the moment generally gets you no where, but if you're going to talk to a host about an incident, you better be able to give a better description than "he was wearing multicam".
Most of the time just give people the benefit of the doubt, if they don't call hit maybe you didn't hit them or maybe they didn't feel it. At Iceback in August I was walking just outside of range for a group that were trying to shoot me, I watched as the BBs hit the ground 20 feet in front of me, yet they were all yelling that I was hit. It is easy to lose sight of a BB after 100 feet, and a lot of players tend to assume if you aim the gun at a guy and pull the trigger it will hit.