I like to use the composite chart to determine the answer to that question. That type of chart is based on the midpoints of both people's planets and ascendant. The purpose is determined by the whole complexion of the new chart but also where the composite sun falls in the chart and its aspects.
Sometimes, more times than not, even with the most difficult relationships, the people involved breath a deep sigh of relief with that knowledge. It seems to free up the person or couple. Sometimes a lot of the tension within a relationship is about where are we going? what is the purpose of this bond? is this short term? or long term? (which translates to: I am scared to be vulnerable)
Last year, I had a very upset client about a break up in her relationship. I knew that was short term as they broke up during the Mercury RX.( They did not meet on a Mercury RX, thus a break up is close to imposiible on a Mercury Rx) i gave her the frameword for his return-date but also the purpose of the relationship...which was clearly marriage. She stopped and looked at me. How can you say that?
I have an invitation to their wedding on my desk. She wrote me a little note. Thank you, with the information you gave me, I was able to relax and relate...Look where I am!
Also composite charts are excellent for diagnosising a problem in a relationship or rather confirming what is happening, and how long the problem will be an issue. This study has to be done on top of a full compatibility to make sense but works for the return client that already has had that work done.
Last and not least, the composite is excellent for group endevaors, revealing their destiny too.