They are male structures
Tibial hooks, more precisely tibial apophyses, are structures some mature male tarantulas develop on the front legs after the maturation moult.
Not every species has them
The presence or absence of hooks varies across genera and species. A mature male without hooks is perfectly possible.
Use them with other signs
Palpal bulbs are another important maturity sign. Body shape, roaming and the known biology of the species provide context.
Do not use hooks to sex juveniles
Juvenile males do not have mature-male hooks. Their absence before the final maturation moult tells you nothing about sex.
Species pages should say when relevant
For species where hooks are a familiar identification feature, the care page can mention them. There is no need to force the field into every quick-facts box.
Record the moult
If a male matures in your collection, record the date. That helps explain the later increase in roaming and reduced appetite.