I do believe eyes are the window to the soul and you can learn a good deal by looking into someone's eyes. But I don't think most people are surprised to find there is more to love than first impressions. While astrology can't tell you if you will fall in love with a person with a particular chart (that depends upon the soul-and your choices) it can give you clues into whether or not the relationship would be easy or difficult, heart stopping or bland, and with the potential for a long term partnership.
With the current epidemic and the growth of online dating, potential mates may be left even more in the dark. In my practice I have found that women are more open to astrology than men, so it may be tricky for women to ask someone for their birthday early on in a relationship. If you can get this information, even without a birthtime, then you will have some clues you can work with to assess compatibility.
Best of Jupiter ties
Jupiter conjunct the other person's Sun, Moon, or Ascendant are the best. The Jupiter person will be like a gift to the other person, and the person who has the planet aspected by Jupiter will return the good energy by simply making the Jupiter person feel good about life.
Sun conjunct Moon
When one person's moon conjuncts the other person's Sun there is a natural connection. Carl Jung wrote a a book called *Synastry* and in it he cites a study showing the commonality of this relationship tie in married couples. I have had a lifelong love of Virgo men, and my moon is in Virgo. Even as a school girl the one father I thought was handsome turned out to be a Virgo.
Sun oppose Moon
Still compatible. The combo ties two people together. It is like looking into a mirror, especially if you have it both ways.
Venus to Mars
When your Venus aspects the other persons Mars, (this includes conjunction and opposition, sextile, trine and square) there is a magnetic and sexual attraction. Without some Venus Mars aspect between the two charts, the chemistry will be missing.
Mercury to Mercury or other key points in chart
Do you want to be able to communicate with this person? Then hopefully your planet of communication is aligned with theirs. This is so important one author (Lois Rodden) wrote an entire book (The Mercury Method of Chart Comparison) on the topic.
My son has Mercury exactly conjunct his girlfriend's Mercury. She is from Mongolia and learning English. In the meantime he picked up Mongolian with no problem!
Aspects to Ascendant/Descendant
The Ascendant and Descendant are the key players in any relationship. One person noted that if the ruler of one person's ascendant or descendant by sign does not match up with the other person's ascendant or descendant ruler by sign, marriage will not be the outcome. Of course you need to have both person's birth times to gauge this one. And of course no single rule will always be true. Often you marry someone who has Sun or Moon on your Descendant.
Aspects to the Nodes
Significant points between one person's Ascendant, Sun, Moon or Venus, Mars, Jupiter or Saturn and the other's persons nodes are key to relationships. The nodes really are all about relationships. The North Node promises room for growth and shows where you are headed. But don't discount the south node. When you meet someone who has south node conjunct a significant planet in your chart, you may be irresistibly drawn to that person. South node pulls you back into the past, but it feels known and comfortable. This may be your soul mate.
Moon Aspects
A whole book could be written about aspects between Moon signs. Read any of Stephen Arroyo's first three books for an in depth look. (Astrology, Karma and Transformation, Relationships and Life Cycles and Astrology, Psychology and the Four Elements).
People with the Moon in the same sign live well together, but I have noted it is often more of a brother sister relationship than a highly romantic relationship.
Ideally you are looking for someone with Moon in the same element as your Moon. Earth moons go well with other earth moons. So your Virgo Moon gets along with those with a Taurus or Capricorn Moon. Water moons (Scorpio, Pisces and Cancer) go well with other water moons. If you have to split it between elements, earth and water are compatible, air and fire are compatible.
However, if you have the natal Sun Moon match, then incompatible moon signs can still work. No one factor ever rules out an entire relationship.
Best to avoid
Mars conjunct Moon or Mars square or Oppose Moon
While this may cause an attraction, the person who has the Mars may hurt the feelings of the Moon person, especially if the Moon person has a vulnerable moon placement. It would be a good aspect for fertility if that is what you seek.
If you have a choice between someone who has Mars conjunct or in a tough aspect to your Moon and someone else, pursue the someone else.
Uranus Square Mars
This can be a very sexy aspect, but unless you both meditate every day, this aspect can lead to fights.
Mars square Mars or oppose Mars
It is hard to work together and while you are first in love there may be no work, but longer term, a relationship is work. A person married to a well known astrologer once confessed to me that this was an especially difficult aspect in their relationship.
Can't end without....
Saturn
Once again, an entire book could be written about this planet in synastry. If you meet someone who has Saturn on your Sun, Moon or Ascendant, this is a karmic tie, and you will feel it. This is the weak in the knees aspect, for good or bad. In the beginning the Saturn person holds all the power in the relationship as the other person can be hopelessly besotted. The person who holds the Sun or Moon will inevitably change and grow and may end up feeling tied down by the Saturn person. But strong Saturn ties usually indicate a long term relationship.