We'll be the first to say that being worried about someone knowing if you've read their message is a little ridiculous. Reading the message is the whole point of texting and yet... we can also admit there are some strategic reasons why you'd want a message to appear unread. Many messaging services like WhatsApp, Facebook Messenger and DMs on Twitter all support read receipts. iMessage-sending iPhone users have read receipts too, with a fun caveat: they're very easy to get around.
In iOS, the read receipts option can be toggled on in the Settings app, under Messages. If you usually keep this option off then you'll never have to worry about sending read receipts, even if the person you're texting has theirs set to "On".
If you usually have read receipts turned on, however, but want to secretly read a message, 3D Touch is your friend here.
In the Messages app, find the person whose texts you'd like to read. From this screen, simply press on their message and apply a little pressure. This will allow you to "peek" into the message, showing the message's entire contents and even previous texts if they went multiple messages in a row.
Be careful not to apply too much pressure though — this will open the message all the way up, sending the dreaded read receipt and letting the sender know that you've seen their message and just opted not to respond.
Keep in mind that the "peeking" into messages trick is only available to those who have an iPhone 6s or later and that the same option doesn't apply to a text's notification banner. When you swipe down or press into a text that just rolled in, this will trigger a read receipt.
This is also a good time to consider that you can change read receipts settings for individual contacts, in case you want Mum and Dad to know when you've read something, but don't necessarily want the person you've just started dating to have that info. Or vice versa — some dates deserve to end on a "Read."