Your sex life is over
As with other flags on this list, a change in sex life alone isn’t necessarily enough to tell you you’re on the outs. But if your sex life sees a drastic turn for the worst and stays that way, it’s time for you to start accepting that the most classic teller of relationship status is speaking volumes.
What you can do: Get back into shape (if you've fallen out of shape), start putting more effort into your appearance, don't apply any pressure on them for at least a week, and drop all clinginess. Busy partners = attractive partners.
They’re defensive and dramatic over ridiculous things
Does you asking a follow-up question create a fight? Do all discussions about politics, holes in Star Wars plots, or Super Bowl picks turn into an argument? If benign conversation consistently sparks controversy and you know less and less about where this person has been hanging out (and with whom), chances are decent your SO is picking fights to justify ways out -- and/or has an increasing interest in something -- or someone -- else.