It's strong, but detect thoughts once per day and a proficiency on Insight might do it.
I thought about this as well, but I feel it has to be at will. The thing is its got to compete with "Dark Ones Blessing" from The Fiend for anyone to even want to pick Great Old One.
Im thinking in terms of making Great Old One at least a viable choice over the Fiend, while being limited to what we have to work with in the scope of the game.
I still think even if you have an at will Detect Thoughts, it still doesnt measure up to Dark Ones Blessing, however I like the idea of tossing him a free insight proficiency as well. Now we're getting somewhere.
I could pick that and feel ok about not getting the dark ones blessing.