Short answer. You raise it by specific denial in your answer. A party who wants to question another party's legal capacity to sue, or their authority to sue in a representative capacity, must do so through a specific denial that includes the supporting particulars peculiarly within that party's own knowledge, not a general or unexplained denial.
What the law says
A party desiring to raise an issue as to the legal existence of any party or the capacity of any party to sue or be sued in a representative capacity, shall do so by specific denial, which shall include such supporting particulars as are peculiarly within the pleader's knowledge.
Rule 8, Section 4 — Capacity. Read the full provision →
Capacity must be averred by the plaintiff first
The rule starts on the plaintiff's side: facts showing a party's capacity to sue or be sued, or authority to sue or be sued in a representative capacity, or the legal existence of an organized association made a party to the case, must be averred in the pleading. A plaintiff who is suing in a representative capacity, or on behalf of an entity, is expected to plead the facts establishing that status as part of the complaint itself, not leave it to be assumed. The same specific-denial route is what the rule prescribes for questioning the legal existence of an organized association made a party.
How a defendant challenges that capacity
If you want to contest the plaintiff's capacity to sue, or their authority to sue in whatever capacity they claim, the rule requires a specific denial in your answer — a general denial that simply says the allegations are untrue will not raise the issue properly. The specific denial has to be more than a bare statement of disagreement; it must include the supporting particulars that are peculiarly within your own knowledge, meaning the concrete facts, from your side, that explain why you dispute the plaintiff's capacity or authority. Supporting particulars peculiarly within the pleader's knowledge is the rule's own phrase — it expects you to plead what you know that the court cannot see on the face of the complaint.
Why the specificity requirement matters
This requirement exists because capacity to sue is a factual question the opposing party is usually best positioned to know something about — whether it is a defect in the plaintiff's registration, the expiration of an authority to act for someone else, or some other concrete circumstance. A vague denial gives the court and the plaintiff nothing to respond to, so the rule ties the right to raise the issue to actually stating, in the answer, the particulars that support the challenge, rather than allowing capacity to be put in issue by mere assertion.