There are several methods to detect vacuum leaks of unmetered air into the fuel delivery system (mechanics.stackexchange.com)
This is usually an indication of a vacuum leak since the percentage of unmetered air entering due to the vacuum leak at higher load decreases and fuel trim improves. (mechanics.stackexchange.com)
Remember you'll need new gaskets to do this or your new engine will have plenty of issues (like vacuum leaks in the intake which will allow unmetered air into the intake system, and exhaust leaks which will allow air into the exhaust track allowing for really crappy gas mileage.). (mechanics.stackexchange.com)