If you open up "Options", you can choose to have it so that non-biological connections use a different connecting line (ex. a dashed line instead of a solid one)
If you want to differentiate between specific non-bio relationships (like foster vs. step) at a glance, you could probably make use of the colour labels (found in each tree member's details section) as well, though you'd have to go through your entire tree and edit each person individually + remember what type of relationship each colour represents
