Astrologically, one clear indicator of a soulmate is someone whose personal planets (sun, moon, Mars, Venus, or Mercury) or ascendant form an aspect (AKA an angular connection) with your karmic line.
Conjunctions are the most compelling aspects for soulmates, but tight (0-3 degree) squares (challenging) or trines (easeful) can also convey karmic connection.
The karmic line is the north and south nodal axis, a 180 degree “line” in an astrology chart. The north node is related to goals and objectives your soul is reaching towards in this lifetime, while the south node shows where you are coming from in previous lives (specifically relevant to this one) and earlier in this life (family circumstances).
People you meet in this life but know from other lives will often have planets located within a few degrees of your south node (forming a conjunction). If your south node is at 23 degrees Aries, for example, and you keep connecting with people whose sun, moon, rising sign, Venus, Mercury or Mars is around 21-25 degrees Aries, you possibly knew each other in a past life.
The planet that connects to your north node also gives some indication of the nature of the previous relationship. If someone’s personal planet or ascendant connects with your north node, you probably have an assignment, karmically, to complete together in this lifetime. If your north nodes themselves form a tight trine, square, or opposition to each other, this, too, can suggest a karmic contract.