I used Spotify during a Sprint promo (when they were my carrier way back when). At one point, I received an email stating that an update to the terms was made. I imagine they still do this, but I guess they could just as easily have stopped, not certain.

A lot of services that I’ve signed up with will announce new terms either via email (quick search of my email lists the most recent ones as Papa Johns, PlayStation, Disney Movie Rewards, Skype, Gamestop) or on next launch of app (Blizzard and Steam being prominent ones off the top of my head). I don’t know many that announce they will in advance though (ie at sign up saying they’ll communicate changes to you), so probably better to be proactive than reactive? I don’t know.