You know or may not know the symptoms. They go from hesitation to lean conditions to engine overheating and many others. An intake leak will not be limited to just the manifold seals. It could be worn throttle shafts, cracked or hard vacuum lines (i.e. V.O.E.S. hose), loose mounts and many others.
S&S Cycle says that the easiest way to check for leaks is to do the following. Fire up the motorcycle and hold it at a steady RPM (2000). Using an aerosol lubricant or cleaner (being careful of painted parts) spray all possible leak points, one at a time.
If the bike revs up or dies when an area is sprayed, it has a leak and needs to be repaired. If the condition persists, double check your work. New gaskets can also be damaged during installation.