The Act on Citizenship of the Slovak Republic does not specify in more detail what specific documents could be used to demonstrate the fulfilment of the condition that a citizen of the Slovak Republic had a residence permit or otherwise registered residence in the state whose citizenship he was acquiring at the time of acquiring foreign citizenship. Likewise, the Act does not specify in more detail what specific documents it is possible to use to prove that a citizen of the Slovak Republic actually stayed in the territory of the state in which he had a residence permit during this time.
In the official form intended for the notification of the loss of the citizenship of the Slovak Republic, it is stated that the notifier submits all documents that can confirm or be helpful in the notification of the acquisition of the citizenship of a foreign country, while the documents proving 5 years of residence in the state whose citizenship he acquires are primarily: a document / confirmation of length of stay / residency issued by a foreign country, resident card, or documents confirming employment or study in a foreign country.
Document confirming employment could be for example an employment contract with the place of work in the state of residency etc. Other documents could be for example documents proving payment of rent, hospitalization in a medical facility etc.
We would like to point out that documents issued by a foreign country must have additional verification, unless an international agreement stipulates otherwise, and they must be accompanied by an officially certified translation into the Slovak language. (Documents issued in the Czech Republic do not need to be translated into Slovak.)
If you are interested, we will be happy to provide you with further legal services related to citizenship.