WhatsApp's blue tick is the easiest way to figure if anyone has read your message or not. Whether the person responds to your message or not is a secondary thing, the sender at least knows the message was read. WhatsApp, however, also gives the option to turn off the blue tick, a feature used by many privacy conscious users.
Before we hold forth with the hack, here's a quick history of the blue tick feature. WhatsApp launched the "blue ticks" in 2014. The blue colour meant the recipient has read the message. Earlier, WhatsApp showed one tick mark when a message was sent which changed to two grey ticks after the delivery.
Now, let's explain the hack. Here, we're assuming you've sent some messages to a person who has blue tick turned off.
Step 1: Send a voice message to the other person on WhatsApp.
Step 2: If the person has heard your voice message, the voice message will show the blue tick.
Step 3: The blue tick shows up regardless the person has turned off the blue tick on the app.
It is worth noting that it's not a new hack. In fact, this error was spotted in March 2018.
While the hack definitely works on Android phones, it's a bit pesky.
That said, WhatsApp has been at the receiving end for quite some time. Jeff Bezos' was phone was also targeted through WhatsApp. Just earlier this week, a Vice report revealed thousands of WhatsApp Groups and their content were indexed by Google.