Food poisoning (or some other vile sickness)
Any kind of super-disgusting, extreme illness will do here. Whether it's a flu so bad you can't get out of bed to throw up in the toilet, food poisoning, or an unsavory case of explosive diarrhea: How your partner deals with you when you're sick is very telling. Screw their sensitive stomach; if he or she isn't down to take care of you, get rid of that person.
If the person you're with doesn't call out from work to stay home and make you better, he or she is not worth your time. You can't marry someone who isn't willing to get through the really crappy (literally) times. Getting you well will be the main priority.
Learning to live together is a milestone all couples come to before they walk down the aisle. And no small part of living together is sleeping together -- I mean actual shut-eye, people.
Sleep is one of the most important things in life; and when you're in a relationship, this means sharing a bed and your sleeping habits. When you love someone, you should be able to figure out how to sleep together so the two of you can both get some zzz's.
If you're an independent sleeper who needs to roll away after cuddle time, clueing your partner in on this shouldn't be the end of the world. Spending the next 50 years freezing all night long because your S.O. is an infamous blanket-stealer will not a happy marriage make.
If you can sleep together in peace, you can live life together the same way.
A major loss
Many of us will experience some kind of profound loss while in a serious relationship. This could be the death of a loved one, a crushing layoff from your dream job, or a big missed opportunity of some kind. Loss is just a sucky inevitability in life. But it's also a great indicator of the kind of partner you've got.
When you're down for the count, can the person you're with pull through and be the rock you so desperately need? Can he or she be your shoulder to cry on and never judge you for your vulnerabilities?
When the shoe's on the other foot and you're being the supportive one, does your S.O. accept your love? Or do you get shut out and pushed away? At the end of this experience, whether you should marry the person you're with should be crystal clear.
If you can't travel with the person you're dating, you might as well end it right now. Who we really are comes through when we're traveling. We're stressed, overwhelmed, and in a place far outside of our comfort zone.
You cannot marry someone until you've crossed state lines with them. It's a huge step to figuring out how this person handles the complications that life brings.
True love means sticking together when things get really tough. Financial hardship is an enormous milestone that can either make or break a relationship. If your partner loses his or her job and works hard to get a new one, you can see how strong and resilient this person is.
If he or she becomes complacent and decides to just live off your kindness, you get a hard dose of the reality you were probably ignoring. If you're going through a tough time yourself, whether your partner steps in to help will put your relationship into perspective.
It doesn't matter how carefully you go through life. The truth is, shit happens. To all of us.
Case in point: I take my birth control pill every single day at 1pm. Regardless, I've had multiple pregnancy scares throughout my adult life. Most couples have gone through this at some point -- and how your partner reacts to the possibility of becoming an unexpected parent will confirm or drastically change your feelings for that person.
This is the ultimate defining moment in a relationship and it happens in many different ways. Perhaps your partner's friend is being rude to you and your partner stands up to them. Maybe you're going through a big career change, and your partner becomes your champion.
For each couple, this moment will be different -- but equally as important.
When a person puts your needs above his or her own and values your happiness just as much as theirs, you know that person is marriage material.