A question about Cobb-Douglas production function

I have a problem of finding the solution to question related to the Cobb-Douglas production function. I don't know whether this is the right forum for me to ask the question. Nevertheless, the question is as follows:

For the Cobb-Douglas production function, $\displaystyle Q = AK^aL^b$, determine how the returns to scale depend on a and b. What special form does the function take in the case of constant returns to scale?

The answer given in the textbook is "Decreasing if a + b < 1, constant if a + b = 1, increasing if a + b > 1". And the answer to the second sub-question is "Q = AK^aL^(1-a) (A > 0, 0 < a < 1).

Can someone explain how I can come to the answers? Thank you very much XD