It is not easy to distinguish between a normal traffic and a DDoS traffic. <br>

>DDoS  can be explained in easy words as - <br>
As a human being, i can only discuss (have a chit chat) things with one person at a time and if 10's of people speaks to me at a same time, then i will not be able to answer any of them and i will be in **not available** state for all of them.

Attackers usually generate the DDoS attack from compromised machines (also known as **bot). It may be case that at the time of writing the answer of your question, my machine may involve in ddos attack to some destination (if my machine is compromised, although chance of its is very less). <br>

>As explained, there is no black and white solution available for controlling (or blocking) ddos attack. 

As explained by @gowenfawr, if ddos attack pattern is of udp, then implementation of URF at the ISP level across the internet, can help in blocking the spoofed traffic and hence help in controlling the ddos attack.