To do this, we start by finding in the product a 6-ring containing a double bond. As shown in the graphic below, that double bond locates for us the atoms that originally were atoms 2 and 3 of the diene. Which one we pick to be 2 and which to be 3 doesn't matter.

Now number the rest of the 6-ring as shown. The two new bonds created by the cycloaddition lie between atoms 4 and 5, and between atoms 1 and 6. Now use curly arrows to break those two bonds. The resulting structures are your starting materials. Here is a first example:

Note that I draw the cyano groups in the dienophile cis- because they were cis- in the product. The next example starts from an unusual looking molecule; nonethless, we look for the 6-ring with the double bond as the key to retrosynthesis:

Again, having located the two bonds created by the Diels-Alder, we use curly arrows to break them, thus identifying the two substances that reacted.

Our final example illustrates the point that sometimes more than one retrosynthetic pathway can be found.

The most apparent 6-ring in the product contains two double bonds. The choice of one to use is arbitrary. Typically, we will try both, and decide afterwards which produces the more reasonable set of reactants. In the next example, I chose the other double bond as my marker:

Of these two, I prefer the first, because it has the electron-attracting groups in the dienophile, whereas the second one puts them in the diene.

A third pathway exists as well. Can you find it - and explain why it does not identify a Diels-Alder reaction that is likely to occur?