They don't keep statistics on that but you can use their data tool and look by state and they will note if the data was incomplete (under reported).
Tables 1 through 5 and Table 7 of Crime in the United States contain statistics for the entire United States. Because not all law enforcement agencies provide data for complete reporting periods, the FBI includes estimated crime numbers in these presentations. The FBI estimates data for three areas: Metropolitan Statistical Areas (MSAs), cities outside MSAs, and nonmetropolitan counties. The FBI computes estimates for participating agencies not providing 12 months of complete data. For agencies supplying three to 11 months of data, the national UCR Program estimates for the missing data by following a standard estimation procedure using the data provided by the agency. If an agency has supplied less than three months of data, the FBI computes estimates by using the known crime figures of similar areas within a state and assigning the same proportion of crime volumes to nonreporting agencies. The estimation process considers the following: population size covered by the agency; type of jurisdiction, e.g., police department versus sheriff’s office; and geographic location.