The only way to attack a federal indictment that charges a conspiracy count based on a lack of evidence is to try the case. "Back in the day," the Government had to prove the existence of a conspiracy for other evidentiary purposes (not relevant to your question, so I won't digress), and if the conspiracy count was weak, that pretrial hearing would usually cause the Government to re-evaluate its case. However, that requirement has all but been completely eviscerated.
