How to Check for Voids in Brazed Joint Quality?

Ultrasonic or x-ray.

Agreed!

Another possibility, depending on what equipment you have available, is Thermal Imaging (Heat one side and take an IR picture of the other side)

Ditto.

Hi, the best solution is to scan ultrasonically from any side of the two sheets with straight beam. If there is metal continuity, the echo will correspond to the sum of the thickness of both sheets. If some misbonding is present, an echo should appear at a distance equal to the thickness of the sheet from whose side you are scanning (or maybe to that tickness plus the thickness of filler metal)

We have always used UT

A peel test can be used if you have a sample of the joint.

Yes, but peel test is destructive and can be used only as "qualification test" to set the process parameters, Which is obviously recommended, but not feasible for quality control of all welds.

thre is a simple test which is available in the market is die penitration test.

there are three solution one is cleaner second one is penetrator, third one is developer.

for this you have to grind the joint and spray the above spray and you will come to know the deffect.

Thank you for you cooperation, but dye penetrant is a NDE technique which allows to detect only surface open defects and in a brazed joint you could only check if the visible part of the weld (obviously minimum) has continuity. You can't detect "voids"