Step 1) Get your Ascendant degree (makes no difference if it is sidereal or tropical, so long as you stick with whichever you choose, for all the steps here)
Step 2) Put that degree (from step 1) into the sign that makes your 7th house, and consider it the degree that will receive the aspects and conjunctions to the 7th house.
example: if 23º Capricorn rises, then 23º Cancer will receive aspects and conjunctions for the 7th house.
Step 3) If a planet comes into proximity of that degree, by conjunction or aspect, you have found an important transit to interpret.
example: if Jupiter comes near 23º of Cancer, Capricorn, Pisces, or Scorpio – you will have an important transit of Jupiter to your 7th house. (If its in Cancer the transit is by conjunction – otherwise, its by aspect)
Note A: You should calculate exact aspects by degree, but as a basic rule of thumb, take 4º on either side of the point (in this example, from 19 ~ 27º of the above signs). That will get you into the ballpark without forcing you to do exact calculations when searching for potential transits.
Note B: You should also consider how much time the transiting planet will stay in proximity of that degree. The longer the duration, the more pronounced the effect of the transit. Thus slower planets have more important transits, but any “station” and beginning or end of a change from retrogression to direct motion will cause any planet to spend longer than usual in a given vicinity.
Note C: Transits to planets are exactly the same, but you don’t have to do steps 1 and 2. Just directly take the planet’s degree as the point that will accept aspects or conjunctions.
Note D: You have a “double transit” (or more) if two (or more) planets affect the degree or range at the same time (in the above examples, the “degree or range” is 19º ~ 23º ~ 27º in Cancer). Double transits are very important.
There are other ways to get a “double transit” too. For example if Jupiter transits your Saturn while Saturn transits your Jupiter.