Checking the percentage of your iPhone's battery can be very important. Especially when you're on the go.  With iPhones previous to iPhone X, their was an option to show such information in Settings > Battery.  However, their is no such option on the iPhone X.  But there are three ways you can still check what percentage your battery is.  Here's how:
Option 1:
Put the device on a charger (wired or wireless).
Option 2:
Ask Siri: "Siri, what is the battery percentage?"
Option 3:
Swipe down from the top to invoke Control Center.  The percentage will appear in the upper-right corner.
That's it.