This is stated in both the IPC-A-610 and the J-STD-001. IPC prescribes that the inspection magnification for cleanliness can take place without magnification (with bare eyes). However, “high density boards” (printed circuit boards on which the components are placed very close next to each other) and with fine-pitch components (SMD components where the leads are very close together), magnification may be necessary. In the tables in the standards you will find exactly which inspection magnification and referee magnification factor is required.