The very first split is on marital status. After that, the next two splits are cap_gain on the left and education_num on the right. Let’s start with looking at cap_gain to see if there is any interaction. If you go further down the splits to the right, cap_gain is used as a split as well. So regardless of the marital status, eventually cap_gain is used as a split. Since the split of marital_status does not change whether cap_gain is used as a split or not, signals to us that their is likely no interaction.
Now lets look at education_num. It is only used as a split on the right branch, not the left. Since this variable is only used when marital status is “no” for the listed categories, this indicates that education_num only affects/matters when marital status is married spouse present, etc (I cant recall the other categories).
This is how I have been thinking about interactions in decision trees.