Approximately 90% of housing in Russia is rented out without formal contracts and, accordingly, without paying taxes on this income, the State Duma has estimated. 

The RIA Real Estate website published an article where Yulia Pavlova, senior tax consultant at Tax Compliance, described the sources from which the tax authorities can find out about such people: 

From bank statements of individuals

The presence of regular payments in equal amounts from one person may indirectly indicate that the person is engaged in business activities. This may be a reason to ask the taxpayer for explanations about the sources of funds (Article 86 of the Tax Code of the Russian Federation).

From Rosreestr data

If a person owns several apartments, this is suspicious for the Federal Tax Service, which has the right to initiate a check of his income. 

From "voluntary" informants

These include management companies, which can provide the tax authorities with information about the leasing of apartments.

In addition, if the rental of premises is carried out to a legal entity, it is the organization that pays personal income tax for the "individual". In this case, the tenant-agent in the 2-NDFL certificate indicates a certain code of income payment. The tax authority, based on this code, selects individuals for possible audits. 

Continued in the source: RIA Real Estate website.