How do you know how strong your Culture is vs Opponents?

For instance, if i want to bombarde an ememy planet with my culture, what's better: having multiple broadcast stations with a lower Culture Tech or having one broadcast station with higher tech? These culture stations are expensive afterall.

And How do you know how many culture stations you need to build on a planet to overcome an enemy's culture?

I wish there was a simple, visual way to determine exactly (where) who's culture is stronger when you zoom out and look at the planets.  It would be immensely helpfull if, say, when your culture was stronger on a planet where you built 4 broadcast stations, the culture "lines" would get thicker or something so you could tell at a glance where the strongest areas of culture were on the map.