A couple of ways: a) with a scale, reading the amounts as it brews, or b) brew into a cup with measuring marks. There’s also another way but it involves some experimentation and memorization – take your favourite cup, pour in 75-90ml of water, note the level. Then add in about 45ml-10% (so 45-4.5, or 40ml) water and note the level it reaches in the cup. That’s your stopping point for any espresso brew.

The -10% thing accounts for crema production and how its volume visually is different from just pouring water.