Regarding your last sentence: Most clients do this automatically, they try to find out which relays are used by the ones you follow and connect to them read-only.
Discussion
This was a helpful comment. Makes it clearer to me, UI wise.
If you are interested in details try running gossip:
https://github.com/mikedilger/gossip
This client allows you to set read & write relays and you can watch on your console what it is actually doing, for example trying to determine the smallest set of relays that covers all your followees.