To start off, the balance needs to be calibrated by a calibration laboratory periodically. To ensure that equipment is fit for purpose, namely the balance in this case; and to provide metrological traceability for a test to be performed; the test laboratory must perform intermediate checks to ensure that the calibration is still valid. This is known as verification which is usually performed on use or on a daily basis. The mass pieces used for verification must also be fit for purpose. This means you should have mass pieces that cover the range of use of the balance and have calibration certificates for the mass pieces. This should indicate that they are fit for purpose, meaning the accuracy and the measurement uncertainty is acceptable. Note that there are various classes of weights as per ASTM and OIML that are matched to the class of the balance. See OIML R 111-1 (E) Edition 2004 available at https://www.oiml.org/en/files/pdf_r/r111-1-e04.pdf. The laboratory must ensure both the balance and mass peices are suitable to provide the resolution and accuracy required.
For more information on associated calibration intervals, refer to ILAC G24:2007 Guidelines for the determination of calibration intervals of measuring instruments (note currently under revision) available for download at https://ilac.org/?ddownload=818