First of all, there are usually three things that lead people to suspect a BOV leak: an audible whooshing sound under boost, a drop in boost pressure, or their mechanic concludes that it is after performing a smoke test.
If there is a suspected BOV related air leak for any of the 3 reasons above, check the o-ring in the base is installed and lubricated, the grub screws are securely tightened and any hose clamps are tightened. Then check that the vacuum hose on the top of the BOV is connected to the manifold with no other accessories tee’d in (see #3). A BOV relies totally on receiving boost pressure from the manifold to stay shut when the throttle is open – if the hose is not connected or suffers a pressure drop for any reason, the valve can open under boost. It’s extremely rare for the workings of a BOV itself to actually leak – leaks usually turn out to be elsewhere in the inlet system – like a loose intercooler plumbing connection or a blown manifold gasket.
Audible whooshing sound: there needs to be a significant leak for it to be audible whilst driving. If the valve installation is checked as described above, look for loose intercooler hose clamps, gaskets etc.
A drop in boost pressure or loss of power can occur for a number of reasons depending on the car. As above, there needs to be a significant leak (the equivalent of a 1 cm / 1/2” diameter hole) for there to be a noticeable boost drop. The only way a BOV can leak this amount is if the top of the valve is not receiving full boost pressure. If it is, then unless the valve is physically jammed open, the leak will NOT be coming from the valve. In the case of late-model Audi and VW engines, sometimes the ECU can reduce boost as a safety measure if it detects something is not right (e.g. if the valve is not adjusted correctly).