Who checks the signatures?

Legal Regime governing Referenda (Law no. 15-A/98 of 3 April 1998, last amended by Organic Law no. 3/2017 of 18 July 2017)

Article 17: Form

4 – The Assembly of the Republic may ask the competent departments and services of the Public Administration to use sampling to conduct an administrative verification of the authenticity of the identity details of the subscribers of popular initiatives.

Legislative Initiative by Citizens (Law no. 17/2003 of 4 June 2003, last amended by Law no. 52/2017 of 13 July 2017)

Article 6. Requisites

5 ‐ The Assembly of the Republic may ask the competent departments and services of the Public Administration to use sampling to conduct an administrative verification of the authenticity of the identity details of the subscribers of legislative initiatives.

The legislation mentions "the competent departments and services of the Public Administration", but there are no further specific details.