An interesting engineering question that needs to be raised is, given no known nuclear test conducted by Israel, how do they know about the reliability of their

An interesting engineering question that needs to be raised is, given no known nuclear test conducted by Israel, how do they know about the reliability of their design ? Assuming they built the warheads by themselves. Did they use foreign blueprints ? Or did foreign countries tested their weapons under their [the foreign country's] official program ? Or do they have access to advanced computation models [which is less probable given they had these weapons since the 70s]