When you say "leaky" are you talking about an injector that's clogged somehow and dripping into the cylinder? Or are you talking "leaky" and spraying or dripping fuel on top of the intake? If it's the latter,...then you need to replace the seal. If it's the a dirty injector you suspect there two ways to test an injectors flow:
Cheap way: Pull the injector and stick it in a glass jar, (while still connected to the fuel rail and electrics). Turn the engine over and watch the pattern. It is possible to measure flow rates, but keeping the injector flowing on a car like the BMW is going to be very tricky electrically. While I've done this on other vehicles, from the looks of the manifold set up on a 4.4, I doubt this is even possible.
Proper way: Remove the injectors from the car, and put them in an injector flow bench. Usually, these benches can both clean and test injectors. Like this:
Fuel Injector Optimizers | C-501 Automotive Optimizer
You may need to find a very well equipped shop that has one of these. The basic version is $1,600...the one linked is $2K.
Alternative Proper Way: Replace the injectors with new ones.
What To Try Before Anything Else: Clean the injectors. Use a system that uses shop air, and force feeds the injector cleaner into the fuel rail, (no gasoline flowing...just cleaner). You can also feed in engine de-carbonizer with the same set up. I recomment the OTC Kit. You will also need their Fuel Injector Test Kit, (so you have the proper adapters to hook up to the car, plus the valves to bleed off the pressure when you're done. You'll also want to test the existing fuel pressure so you can match it with shop air.)
Here's the OTC Injector cleaning Rig:
Fuel Injection Testers/Kits OTC 7448 OTC7448 OTC TOOLS 7448 - Fuel Injection Canister Cleaning System
The OTC Cleaner:
Fuel System OTC 7000A OTC7000A OTC TOOLS 7000A - Fuel Injection Canister Cleaner, Case
OTC Fuel Injection Test Kit:
Fuel Injection Testers/Kits OTC 6550 OTC6550 OTC TOOLS 6550 - Fuel Injection Master Adapter Kit
Decarbonizer:
Motorvac 400-0060 Motorvac Corp 400-0060 - (6) CarbonClean MV-6 Maximum Strength Cleaner – 16oz Cans
I have used the above set up many times, on a number of different vehicles. It almost always make a car run better; sometimes very significantly. It does NOT always solve a car's problems. If the problem really is a clogged injector, there's a pretty good chance this will work. If an injector is faulty....no.
Also, there are considerably more sophisticed versions of the above linked OTC machne. Like this $1,600 Snap-on machine:
EEFS505A, Fuel System Decarbonizer
But it does exactly the same thing as the OTC rig, and you're still going to need adapters. Snap on also sells a similar set up to the OTC, (at about 3x the price). I am not fond of this Snap-on system as it mates with their Fuel Injector Test Set. The OTC set has many more adaptors. The OTC is basically all brass stuff. Much of the Snap-on adaptor sets are plastic. I've owned both, and sold my Snap-on set on eBay. You pretty much need to get your test set and cleaning rig from the same vendor; they need to be used together.
There are also similar machines that claim to atomize the cleaner before forcing it in the engine. I have not see one of these in the US, but Ed China used one of these on a recent episode of Wheeler Dealers. Better than the above systems? I can't really say. But all these injector cleaning systems really do work.