First, I believe it is more about what doesn't happen in a relationship than what does happen. I hope there will always be a dash of mystery and magic in love relationships for without those ingredients it can become an intolerable burden to bear.
Unfortunately, I've had the pleasure of observing many mismatched couples, and the dicey circumstances of being one-half of such a couple on one long occasion and a couple briefer ones. It wasn't those guys were so bad, they were simply wrong for me. Well, actually they did have some hopeless pathos such as passive aggressiveness, infidelity and pathological lying but hey let's not start nitpicking here!
It is all about discovering and acknowledging the uncomfortable fit before too much time is invested. I have to admit I stayed in one relationship for more than 20 years and I have zero happy memories of us as a couple. We were day and night, an optimist and pessimist, an extrovert and an introvert. One was social the other a loner. So why did I stay so long, get ready to laugh but remember I was young--I thought he would change. Yep, sure did. But he sure did not!
So trust me, this is the voice of experience speaking and here are the signs and feelings you should not ignore, and the questions you should answer early in the relationship:
1. Do you enjoy talking with him and he with you? It was on the honeymoon I realized my marriage was going to be a lonely undertaking, and it was a mistake from day one. Once he was no longer trying to win me he slipped back into the silent shell a few people warned me about. There was very little communication and he was only affectionate when he was in the mood for... well you get the picture.
2. Who is the first person you want to share good and bad news with? If it is not your special someone then perhaps you should reconsider your choice of a life partner.
3. Are you generally happy to see your special someone walk in the door, or do you secretly enjoy the time without him/her? I noticed my mood would darken as the dinner hour approached. I used to blame it on low blood sugar but I think my endorphins went into hibernation as the homecoming hour approached. Those feel good feelings were gone.
4. Is he/she willing to change his/her life because of yours? This is a biggie---it takes a contortionist to always wrap their life around someone else's, and with time, that gets pretty uncomfortable. Make sure there is just as much give as take on both sides of the relationship fence.
5. Do you make each other laugh? This may seem insignificant but it is a wonderful test of the relationship. The best marriages and long term relationships I've observed are formed between partners who do a lot of laughing together. They "get one another" instead of wondering "who the hell is that stranger beside me in my bed."