I think the code you provided is the fastest that you are going to get:
is_square n = sq * sq == n
where sq = floor $ sqrt $ (fromIntegral n::Double)
The complexity of this code is: one sqrt, one double multiplication, one cast (dbl->int), and one comparison. You could try to use other computation methods to replace the sqrt and the multiplication with just integer arithmetic and shifts, but chances are it is not going to be faster than one sqrt and one multiplication.
The only place where it might be worth using another method is if the CPU on which you are running does not support floating point arithmetic. In this case the compiler will probably have to generate sqrt and double multiplication in software, and you could get advantage in optimizing for your specific application.
As pointed out by other answer, there is still a limitation of big integers, but unless you are going to run into those numbers, it is probably better to take advantage of the floating point hardware support than writing your own algorithm.