I think theres no way to know if content is on usenet but not getting found through your indexer, apart from trying another indexer and noticing the file is there. I’d suggest getting 2-3 lifetime memberships and they should cover your needs. Sometimes lifetime is available fairly cheap, sometimes it’s not open and you have to wait. In terms of recommendations, I like nzbgeek, drunkenslug, miatrix, and nzbplanet.