DNS Reports clearly list the number of timeouts and errors in your network.
DNS Reports
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Response Code
FORMEERROR/QUERY: A
FORMEERROR/QUERY: SOA
NOERROR
NOTIMPL/OTHER
NXDOMAIN/QUERY: A
NXDOMAIN/QUERY: AAAA
NXDOMAIN/QUERY: NS
NXDOMAIN/QUERY: PTR
NXDOMAIN/QUERY: SOA
NXDOMAIN/QUERY: SRV
NXDOMAIN/QUERY: TXT
SERVFAIL/QUERY: A
SERVFAIL/QUERY: TXT
SERVFAIL/QUERY: AAAA
SERVFAIL/QUERY: CNAME