This is not so easy to answer. In the case of a potential '3rd party', you would first have to delineate the significators for yourself and the person you are with, e.g 1st and 7th rulers. Then I would suggest looking for aspects between your partner, the 7th house and the 12th house, which is the house of 'hiding and secrets'. Barring this, you can take the moon as yourself, and Venus for the '3rd party' as Venus rules sweethearts or lovers. But aspects to and from the 12th will be your key.
If you want to complicate matters you can always try turning the chart, so the 5th from the 7th for your partner's 3rd party lover; but I'd shoot for the easiest delineations first, and *then* try turning the chart.