Scroll all the way to the right, and look to see if there is an edge which is labeled as spanning position 0 (the beginning of the sentence) to n, where n is the number of words in the sentence. (The first thing in an edge label is information about the portion of the string it spans.)
Alternatively, click on each of the words (at the far left edge of the chart) until they are all highlighted. Then scroll to the right to see if any edges are highlighted. An edge that spans the whole chart wil be.