5 C.F.R. § 2422.10
How do you challenge the validity of a showing of interest?
(a) Validity. Validity questions are raised by challenges to a showing of interest on grounds other than adequacy.
(b) Validity challenge. The FLRA or any party may challenge the validity of a showing of interest.
(c) When and where validity challenges may be filed. Your challenges to the validity of a showing of interest must be in writing and filed with the Regional Director or the Hearing Officer before the hearing opens, unless you show good cause for granting an extension. If no hearing is held, you must file challenges to the validity of a showing of interest before action is taken under § 2422.30.
(d) Contents of validity challenges. Your challenges to the validity of a showing of interest must be supported with evidence.
(e) FLRA investigation and Decision and Order. The FLRA will conduct an investigation if deemed appropriate. An FLRA determination that a showing of interest is valid is not subject to collateral or direct attack. If the FLRA finds that the showing of interest is not valid, the FLRA will issue a Decision and Order dismissing the petition, or denying the request to intervene.