It’s very difficult to do, actually, since getting a line-of-sight to the top of a tall tree in a forest is next-to-impossible (having tried). So while you can do the whole laser-sight to the top and measure to the base and use trig to figure the height, most of the "world’s talles tree" measurements these days are taken by climbing.
A lot of historic tall tree measurements were taken from cut trees–which begs the question, are trees taller lying down than standing up, without their own weight pulling them down (like you’re minutely taller in the morning)?

It’s done by triangulation. You need to know the angle of the corner you are measuring from and the length of the hypotenuse. There are IF and laser measuring devices for that.