The general rule is that appeals wait for a final judgment, which prevents cases being interrupted repeatedly. Several routes exist around it, and each is narrow.
Orders appealable as of right. Certain categories — commonly orders granting or denying injunctions, and some orders concerning receivers or class certification — may be appealed immediately by rule or statute.
Certification by the trial court. Where an order involves a controlling question of law on which there is substantial ground for difference of opinion, and an immediate appeal may materially advance the termination of the litigation, the trial court may certify it and the appellate court may, in its discretion, accept.
The collateral order doctrine. A small class of orders that conclusively determine a disputed question, resolve an important issue separate from the merits, and would be effectively unreviewable after final judgment. Denials of certain immunities are the classic example.
Mandamus. An extraordinary writ, available where there is no other adequate means of relief and the right to it is clear and indisputable. Rarely granted.
Practical judgement. Interlocutory review is slow, expensive and usually refused, and pursuing it can antagonise the court that will try the case. It is worth attempting where the ruling effectively ends the case or where litigating to judgment under a plainly wrong legal standard would waste years.