I pressure test headers as I go. Check each tube as you finish weld (use the same parts you use to back-purge the welds).
If you wait and pressure test when you're done, you may find a leak in a spot you can't reach with your welder.
When the heads are done, just clamp them to a flat surface and use compressed air to provide 5-20 psi. Again, just use the adapters you used for back-purging the welds. Even 2psi will be adequate for finding leaks.
Soapy water will find any leaks even if you can't achieve a tight seal on your test scaffolding.
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