The easiest way, and probably the only way is to go through your local police. Like I mentioned earlier, the firearm may be reported lost already. So, I would turn it in. When the police run the serial number, they will know. If already reported they can get it back to the owner. If not reported, they will hold it and hopefully the owner will make a police report. If they do not make a police report, eventually the BATF will identify the owner. The BATF will then contact the agency that is holding the gun and have them contact the owner. The BATF will not do anything unless it goes through the local guys.