Every instruction can request a test for equal or unequal tag, and cause a maskable interrupt if the specified match fails.
There is no architectural connection between the tag and the contents of the half-word; it is strictly determined by the software.
In addition to this, the original Xerox Smalltalk implementation used the least-significant bit of each 16-bit word as a tag bit: if it was clear then the hardware would accept it as an aligned memory address while if it was set it was treated as a (shifted) 15-bit integer.
Current Intel documentation mentions that the lower bits of a memory address might be similarly used by some interpreter-based systems.
In the Soviet Union, the Elbrus series of supercomputers pioneered the use of tagged architectures in 1973.