Some people are so clueless as to the status of their own relationships. Dating someone or marrying them shouldn't be a painfully long slog through nothingness. That person should give you joy, laughter, pleasure, a feeling of connectedness, and peace.
1. You have no peace in your life
From the moment you get up, until the moment you lay your head down, it seems like all you do is fight or argue with your significant other. Every phone call is an angry rant or tirade about something. You absolutely dread coming home because you know something will set you or them off, and you just feel absolutely exhausted by being around this person.
2. Their actions are adversely effecting your life
Your partner should lift you up. His/her or your dreams and goals should be something you're both keen on seeing accomplished and/or doing, but when it gets to a point where everything they do is negatively affecting your life, that's a problem. They drink, they end up in the hospital. They spend money, they go straight into debt. Their anger issues are spilling into every aspect of your life. Their now year long unwillingness to get a job is putting a heavy burden on you to pay for everything. If you've had the dreaded talk, and nothing changes, and they continue on a negative path, it's probably not going to get any better.
3. You are not important to them
You're gone for 2 weeks, and they don't call or text or seem like they miss you at all. When you mention something really important you want them to attend, they always flake on you and come up with dumb excuses why they weren't there. They don't seem to ever call you to let you know they'll be late or let you know where they are in general. You do something incredibly nice for them, and they barely bat an eyelash.
4. They refuse to open up about anything
If this is someone you claim to trust and vice versa, they and you, should be able to share, some, if not most of the aspects of your lives with each other. If you know next to nothing about them and it's been months, and there isn't something deeply psychological going on, this is probably going to cause you a lot of trouble down the road because either they are purposefully being secretive, or they will never trust you enough to share who they are on anything other than a surface level you have with acquaintances.
5. This is the second time they've cheated on you
Some people can forgive a cheater once...but twice?!? One does not and should not get unlimited free passes to cheat on you. If they cheat, and you found it somewhere in your heart to forgive them, that comes with the expectation that it won't ever happen again. If it does, what are you sticking around for unless you simply enjoy being hurt by this person, because you forgive again, what are the consequences for that person who knows, you will probably always take them back no matter what. That's on you.
6. Life in the bedroom is bad
Toughie to admit this one, but if life in the bedroom is what matters to you in anyway, odds are, if things are just bad there, and it's been a while, it's not going to get better. You like what you like, you know what works for you and what doesn't, and if your partner just isn't doing anything for you in this department, odds are, you'll end up stuck and wanting and may end up cheating just to fulfill your needs.