There was an official build from June 20 filenamed lineage-14.1-20180620-nightly-FP2-signed.zip.
Apart from that, I guess everything not containing “unofficial” in Settings - About phone - LIneageOS-Version or “test-keys” in Settings - About phone - Build number would strongly hint to official … but in the end you would have to know what you are running on your own phone 
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