The first place to look is the information page for your monitor in the manufacture’s site. Another place is on a site called Display Specifications (https://www.displayspecifications.com). You should be able to find the maximum brightness performance on these two resources.

Or, if you have a Spyder X2 Elite or a Spyder X2 Ultra versions of the Spyder software, you can use the Colorimeter function in the software. Set your brightness to maximum then measure the Colorimeter target to see the maximum luminance of your display.

A high brightness monitor comes with a brightness value over 400 nits (cd/m^2).