There is no way to be sure that a carefully built doesn't have an air leak. It is possible you have done everything absolutely right but...
after you have built up inside pressure, (this pressure has to be determined ) you put some soapy solution in strategical parts. Manifold, seals... if you see something like in this picture it means air goes out. But if air goes out it means that air can come in during depression, and this is not what an engine needs to work well.
As we said at the beginning, you think you have done everything right? Yes maybe you did, but this apparently perfect seal showed a problem and had to be replaced. It happens that a carefully built engine has to be opened more then once for bad parts. it happened to us to have changed three time the same seal to solve an air leak. You can't be sure before testing.
There is no way to be sure that a carefully built doesn't have an air leak. It is possible you have done everything absolutely right but...
after you have built up inside pressure, (this pressure has to be determined ) you put some soapy solution in strategical parts. Manifold, seals... if you see something like in this picture it means air goes out. But if air goes out it means that air can come in during depression, and this is not what an engine needs to work well.
As we said at the beginning, you think you have done everything right? Yes maybe you did, but this apparently perfect seal showed a problem and had to be replaced. It happens that a carefully built engine has to be opened more then once for bad parts. it happened to us to have changed three time the same seal to solve an air leak. You can't be sure before testing. 
