Total cost function and (dis)economies of scale
First of all, knowing the cost function is not enough to know whether we have decreasing, increasing or constant returns to scale. Returns to scale are a property of the firm's production function -- and the relationship of the production function to the cost function is not as straightforward as you might think. However, if we assume that the firm takes factor prices as given, then we can apply a (rather intuitive) result from Sandmo (1970):


  Average cost is decreasing, constant or increasing according as returns to scale are increasing, constant or decreasing.


In light of this, we simply need to consider the average cost curve:

$$ AC(Q) =100+20Q+3Q^2 $$

Clearly, this is strictly increasing in $Q$. Therefore, if we are willing to assume price taking in the factor markets, we know that the firm's underlying production function exhibits decreasing returns to scale.