SpeechSynthesis.pending

var synth = window.speechSynthesis;

var utterance1 = new SpeechSynthesisUtterance('How about we say this now? This is quite a long sentence to say.');
var utterance2 = new SpeechSynthesisUtterance('We should say another sentence too, just to be on the safe side.');

synth.speak(utterance1);
synth.speak(utterance2);

var amIPending = synth.pending; // will return true if utterance 1 is still being spoken and utterance 2 is in the queue