Linda's Sun in Libra was out of sign closely orb conjunct Paul's Ascendant in Virgo. Paul's Moon was sign and orb conjunct Linda's Midheaven.

Linda's North Lunar Node was conjunct Paul's Ascendant. One interpretation is that this one single indicator by itself strongly denotes that the two of them may have been together during several different past lives. Only mildly of interest: Linda's North Lunar Node was squared Paul's Sun. Linda's North Lunar Node was also sextile Paul's Venus. Paul's North Lunar Node was trine Linda's Venus (a double whammy).

3. You Are My Destiny (Destiny's Gate)
Cross aspects between the Sun, Moon, Venus, Ascendant, and/or Midheaven in person A's chart and person B's birth Vertex, the Vertex being nicknamed by me as "Destiny's Gate." This rule holds especially true in regard to conjunctions and oppositions. Again, look for a double whammy.

Linda's Vertex was conjunct Paul's Sun and Midheaven which was a very powerful connection between the two of them. Only of mild secondary interest, Paul's Vertex was trine Linda's Moon and Venus.

4. Let Me Lose Myself in You! (or Calgon, Take Me Away)
Cross aspects between person A's birth Neptune and person B's birth luminaries, personal planets (Sun, Moon, Mercury, Venus and Mars), and Ascendant/Descendant axis....

The conjunction, trine and sextile aspects are (for the most part) a wee bit "easier" to handle - with the conjunction being the most intense connection.

Cross aspects involving Neptune and the other person's luminaries, personal planets, and Ascendant/Descendant axis can even hold true with sign  conjunctions and oppositions that aren't necessarily within orb aspect - but generally speaking: the closer, the better.

Finding what's sometimes called a "double whammy" cross aspect - such as person A's Neptune conjunct person B's Venus and person B's Neptune sextile person A's Venus -- is all the better for creating that special connection feeling. A double whammy helps create a stronger mutuality of feeling.

In a synastry birth chart comparison for Paul and Linda McCartney - Paul's Neptune was found to be conjunct Linda's Sun; and Linda's Neptune was found to be squared Paul's Sun (a double whammy). Linda's Neptune was also conjunct Paul's Ascendant.

5. Hey, Venus! (Love in the Afternoon)
Cross aspects between the Sun and/or Moon in person A's chart and person B's birth Venus.

This is especially intense with conjunctions and oppositions, but trines and sextiles are awfully comfortable and nice. Again, look for a double whammy.

Paul's Venus was in opposition Linda's Moon and Linda's Venus was squared Paul's Moon (another double whammy).

6. Immortal Lovers (Ooh baby, ooh baby)
Now here's one you probably haven't heard of before...

The Immortal Lovers clue concerns the minor asteroids named Eros and Psyche. If this is a relationship between a male and female -- then first look for the male's birth Eros conjuncting the female's birth Psyche. This even appears to work with sign conjunctions which aren't in orb. In my experience, finding this cross aspect between lovers is fairly rare... at the same time, finding a double whammy between family members seems to be fairly common (which is sort of a spooky thought, but consistent with many of the differing theories on reincarnation).

Bringing it down a notch, this also seems to work fairly well with (in orb) aspect oppositions and squares. Then, to a much, much, much lesser degree, it can work with trines and sextiles. (Although, as a strong general rule I don't consider trines and sextiles when comparing Eros and Psyche.)

Then... check to see if anything's up between the female's birth Eros and male's birth Psyche. Finding a double whammy cross aspect between lovers (involving conjunctions, oppositions and squares) is rare...

Paul's Eros was in aspect opposition Linda's Psyche and Linda's Eros was sign conjunct Paul's Psyche (another double whammy).