The witness is speculating as to the content of the seized packages. 

【翻译】证人正在推测被查获的包裹的内容（所装物品）。

speculate 在本句用作不及物动词，意为“猜测；推测；预测”。as to = about

speculate as to = speculate about：对......作出猜测/推测/预测。

He can only speculate as to which horse will win the race. 

他只能推测哪匹在赛马中会获胜。

 It's always hard to speculate about the future. 

未来一向很难预测。

来自柯林斯例句