Two common tests are (1) to put the device or component under pressure using regulated compressed air and then submerging in water or other fluid. The leaks will show as bubbles or (2) use a colored die solution that contrasts with your part colors under pressure and the die will highlight leaks.
Ultimately whenever possible pressure testing of the final assembly under the working load or more, in as close to the final environment as possible can help identify failures caused by condition stack up.
Alternatively for many applications there may be published standard test methods that have been shown to be effective. Medical devices and aerospace both have test standards and you may find some relevant test standards under ASTM.
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