Well, you could datalog it and check your short term fuel trims to see if there is a leak.
3. So now that we know we're good on knock, lets check our fueling using a combination of AFR / Lambse / Short term fuel trims and long term fuel trims. Its a lot to take in, so lets try to break them into useful pairings again.
For those that don't know, Short term fuel trims are instantaneous corrections your computer is making to your base MAF (mass air flow sensor) curve to correct for any anomalies. Since they're instantaneous, I ALWAYS have my computer and datalogger hooked up any time I do anything major in the engine bay and monitor STFTs when I start the car. This is an excellent way to see if you have any vacuum leaks, or if the car is otherwise unhappy. Excessively positive STFTs (typically around 30-40%) are a dead indication that you have a leak somewhere, and can prevent you from driving around under dangerous conditions.
LTFTs on the other hand, are stored averages of the STFTs for given breakpoints. We don't have access to these breakpoints since we don't know how the tune was written, so for now we're just concerned with the values. In my personal application I view it like this:
+/- 2% is good
+/- 5% is acceptable
+/- 8% needs adjusting
Taken from: How to: Understand your datalogs