My friends all ask the same question. For openers, being 70 is one of the ways you know. DIG: A 70-year old can look back on life (got LOTS of decades to examine) and compare it to everything else they have already experienced. How does it rank? If you can tell yourself it doesns't feel like anyting you've ever felt before, you have either been bitten by a pesky flying pest, or you are in love!
It's not at all like marrying someone because you get along well and feel "comfortable" with each other. It's about how "UNcomfortable" every moment is without being in his presence. The loss is brief when he merely steps out of the room, but no less profound than a whole excruciating day of his absence.