Well, honestly, the only definitive way to do this is to cut the frame apart, making sure that you have full braze penetration on all joints.

Otherwise, you need to look for signs of complete braze penetration, i.e. brazing material on the shorelines of the outside and inside of your bottom bracket, top and bottom of your head and seat lugs, etc. The time you spend getting up close and personal with your frame with emery cloth and files should allow you to check this.

Pete