Hi Cat,
There's loads to look for, and then you need to synthesize it all down into a story. Looking at Sun signs only gives you part of the story, because the way you live your Sun will be modified by all the other elements of the chart.
First, you look at how the two individual charts interact. I tend to look more at what is happening with the planets than at the houses. Aspects to the Sun, Moon and Venus of each individual chart are important. Saturn can be very important. If Saturn from one chart forms an aspect to a personal planet in the other chart, that is an indication of stability and of the personal discipline that is needed to keep a relationship going. However, Saturn is also the planet of restriction and of saying No. So a bad aspect of Saturn with the other person's Venus, for example, is going to throw up obstacles to the relationship. Aspects between personal planets of one chart and the outer planets, especially Pluto, of the other chart can also be important. These sorts of aspects often make the people concerned feel that the relationship was fated long before they met.
Then you can look at a combined chart, which takes the midpoints of all the planets in the two individual charts. This is, in effect, the chart of the relationship seen as an entity in itself.
To all this, I add a study of the individual charts. This is where I tend to pay more attention to houses. The fifth is about flirting, romance and instant attraction. The seventh is about the establishment of relationships. The eighth is about long-term relationships, marriage and also sex. I also look at the second house, because issues about money can so often be obstacles to a good relationship.
To be honest, analysing a relationship is not easy. It takes me many hours to prepare such a report.