One of the most common ways to find a vacuum leak is by using a smoke machine. This machine injects a small amount of smoke into the engine bay, and you can then use a visual inspection to see where the smoke is coming from. If there is a leak, the smoke will be drawn into it and you should be able to see where the leak is located.
Another way to find vacuum leaks is by listening for them. A small leak will often make a hissing sound, so if you hear this while the engine is running then it's likely that there is a vacuum leak somewhere. Another tell-tale sign of a vacuum leak is if the engine idles erratically or stalls when you come to a stop. This is because the engine isn't getting enough air through the intake, so it's worth checking for leaks in this area first.
If you suspect there may be a vacuum leak but you can't find it, then it's worth taking your car to a mechanic who has experience with this issue. They will likely have access to specialist equipment that can help locate even the smallest of leaks.