How to tell iPhone X apart from XS?

I know I'm late to the party, but the model number will also tell you which is which. They've hidden it under the SIM card on the iPhone 8 and later models. From the Apple Support web site:

On an iPhone 8 or later, to find the model number, remove the SIM tray and look in the SIM tray slot. Make sure that you're in a place with bright light overhead. The model number is on the upper side—the side with the display.

Just for fun, here's the list of model numbers for all of the models up to now, August 2022, straight from the Apple web site.

iPhone 1st Gen:
A1203

iPhone 3G:
A1324, A1241

iPhone 3GS:
A1325, A1303

iPhone 4:
A1349, A1332

iPhone 4s:
A1431, A1387

iPhone 5:
A1428, A1429, A1442

iPhone 5c:
A1456, A1507, A1516, A1529, A1532

iPhone 5s:
A1453, A1457, A1518, A1528, A1530, A1533

iPhone 6 Plus:
A1522, A1524, A1593

iPhone 6:
A1549, A1586, A1589

iPhone 6s Plus:
A1634, A1687, A1699

iPhone 6s:
A1633, A1688, A1700

iPhone SE (1st generation):
A1723, A1662, A1724

iPhone 7 Plus:
A1661, A1784, A1785 (Japan)

iPhone 7:
A1660, A1778, A1779 (Japan)

iPhone 8 Plus:
A1864, A1897, A1898 (Japan)

iPhone 8:
A1863, A1905, A1906 (Japan)

iPhone X:
A1865, A1901, A1902 (Japan)

iPhone XR:
A1984, A2105, A2106 (Japan), A2107, A2108 (China mainland)

iPhone XS Max:
A1921, A2101, A2102 (Japan), A2103, A2104 (China mainland)

iPhone XS:
A1920, A2097, A2098 (Japan), A2099, A2100 (China mainland)

iPhone 11:
A2111 (Canada, United States), A2223 (China mainland, Hong Kong, Macao), A2221 (other countries and regions)

iPhone 11 Pro Max:
Model numbers: A2161 (Canada, United States), A2220 (China mainland, Hong Kong, Macao), A2218 (other countries and regions)

iPhone 11 Pro:
A2160 (Canada, United States), A2217 (China mainland, Hong Kong, Macao), A2215 (other countries and regions)

iPhone SE (2nd generation):
A2275 (Canada, United States), A2298 (China mainland), A2296 (other countries and regions)

iPhone 12 mini:
A2176 (United States), A2398 (Canada, Japan), A2400 (China mainland), A2399 (other countries and regions)

iPhone 12:
A2172 (United States), A2402 (Canada, Japan), A2404 (China Mainland, Hong Kong, Macao), A2403 (other countries and regions)

iPhone 12 Pro:
A2341 (United States), A2406 (Canada, Japan), A2408 (China mainland, Hong Kong, Macao), A2407 (other countries and regions)

iPhone 12 Pro Max:
A2342 (United States), A2410 (Canada, Japan), A2412 (China mainland, Hong Kong, Macao), A2411 (other countries and regions)

iPhone 13 mini:
A2481 (United States), A2626 (Canada, Japan, Mexico, Saudi Arabia), A2629 (mainland China), A2630 (Armenia, Belarus, Armenia, Belarus, Kazakhstan, Kyrgyzstan, Russia), A2628 (other countries and regions)

iPhone 13:
A2482 (United States), A2631 (Canada, Japan, Mexico, Saudi Arabia), A2634 (mainland China, Hong Kong, Macao), A2635 (Armenia, Belarus, Kazakhstan, Kyrgyzstan, Russia), A2633 (other countires and regions)

iPhone 13 Pro:
A2483 (United States), A2636 (Canada, Japan, Mexico, Saudi Arabia), A2639 (mainland China, Hong Kong, Macao), A2640 (Armenia, Belarus, Kazakhstan, Kyrgyzstan, Russia), A2638 (other countries and regions)

iPhone 13 Pro Max:
A2484 (United States), A2641 (Canada, Japan, Mexico, Saudi Arabia), A2644 (mainland China, Hong Kong, Macao), A2645 (Armenia, Belarus, Kazakhstan, Kyrgyzstan, Russia), A2643 (other countries and regions)

iPhone SE (3rd generation):
A2595 (United States, Canada, Puerto Rico, U.S. Virgin Islands, Guam, Mexico, Saudi Arabia), A2782 (Japan), A2784 (Russia, Armenia, Belarus, Kazakhstan, Kyrgyzstan), A2785 (China), A2783 (other countries and regions)

You can also tell by the antenna line on the bottom next to the speaker holes, the iPhone X doesn’t have one at the bottom. :-)

The only way to tell is the screen take the screen from an x and xs and you will see different mounting hole for the earpiece speaker the screw studs are different places and the screen attachments on the side and top of the screen are different places. You might also check the sim card tray and see if it has the imei number on it.