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To check for a vacuum leak, simply just open the oil filler cap on the valve cover while the engine is running. If the engine makes attempts to keep the idle constant then you probably don't have a vacuum leak. If the engine does not try to keep the idle constant then you do have a vacuum leak, because the engine is constantly trying to maintain idle under a vacuum leak condition.
If it does have a vacuum leak, take it to a BMW independent shop that can perform a smoke test. Most good auto shops have a smoke test machine, which is a tool that adds smoke into the vacuum system and will show if and where the various vacuum circuit of the engine have a leak.
As far as the check engine light, even though it is off, if it came one at one point, the event is stored in the ECU (engine computer) so the Technician can diagnose the problem. If it is a vacuum leak, then the codes are most likely lean mixture codes that go away once the vacuum is restored.
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