To check the results of the synchronization you need to make sure that Highlight lyrics during playback option is checked (this corresponds to pushed in Highlight during playback button on the lyrics pane toolbar) and play the file. As the file is played back, the lyrics that should be sung are highlighted. Note that the words are highlighted letter-by-letter, so the singer can see how fast should the word be sung. During the playback you may move the cursor and the playback will resume from the selected word.