There are very expensive testing devices used by large companies. You don't have access to these. So all you can do is miter each tube perfectly, ensure that all mating surfaces are within tolerance, perfectly clean, and thoroughly fluxed. Then you have to heat the joint evenly, ad filler from one spot and pull it through to every other spot. After removing the flux you closely examined every joint under strong light and ensured that there is indeed filler material showing evenly at every shoreline, and from the miters inside the bottom bracket shell, etc.

If you did these things with every joint, your frame is strong and safe. If you did not, it is not.