You will find a pressure gauge on the fascia panel of your boiler. In modern combination boilers, you will see digital calculations with a flashing pressure reading if there is a warning of high or low pressure. In hydraulic gauges, you will see the needle in the red zone if there is a warning.

If you see the needle at the reading above 2.5 bar, your boiler pressure is too high. If the needle is below 1, your boiler pressure is too low.

Image Source: https://247homerescue.co.uk/blog/boiler-losing-pressure/