I looked around a little and it turns out that there is a bitfield, byte ident, in the struct object_type dealing with the state of identification an item has. The byte is the datatype.
* Special Object Flags
#define IDENT_SENSE 0x01 /* Item has been "sensed" */
#define IDENT_FIXED 0x02 /* Item has been "haggled" */
#define IDENT_EMPTY 0x04 /* Item charges are known */
#define IDENT_KNOWN 0x08 /* Item abilities are known */
#define IDENT_STOREB 0x10 /* Item is storebought !!!! */
#define IDENT_MENTAL 0x20 /* Item information is known */
#define IDENT_CURSED 0x40 /* Item is temporarily cursed */
IDENT_SENSE is the pseudo-id flag. It gets turned on when the object is pseudo-id'd. In ToME 2 this happens randomly when you carry stuff around, but I don't think it's hardcoded in the T-Engine. Instead, module code can flip the bit if the module maker feels like including the feature.
Also in object_type is "byte sense" which is a number which determines which description is picked from the sense_desc table (in tables.c) that has the different things objects can be pseudo-id'd as.