The “pythonic” way is to go with what’s readable and maintainable.
That said, x == float("inf") and x == float("-inf") are slightly more readable to me, and I’d prefer them. math.isinf(x) and x > 0 is faster, but only on the order of about 40 nanoseconds per call.
So unless you’re checking a whole lot of numbers, it isn’t going to make much of a difference in running time.