I know I worked a person because they're in my log. LOTW and eQSL are just ways of publishing that log, as far as I'm concerned. You don't, by the way, know beyond the shadow of a doubt that you worked the person just because they've uploaded an entry to LOTW. I know for a fact of at least one entry in LoTW for a contact that was never made but that both parties have confirmed. LOTW is a fairly good example of things that go wrong when people take amateur radio too seriously and trust each other too little. It's over-engineered, unnecessarily "secure", and under supported. It could have been a true logbook of the world, in which amateurs cooperated with each other to put together a shared history of QSOs, to clean up log errors, and so forth. As such it could have become a wealth of information about propgation conditions over time, the history of our activity, and so forth. Instead, mistrust and meanness have made it into a complex piece of crap, where honest errors can't be corrected, that requires far too many hoops to go through to be worth using. I tried it for a while; but since I'm not chasing DXCC, I find the annoyance outweighs the advantages and I let my certs expire.