how can you tell when a female tarantula is mature?

[auskie]

I believe I have a four year old female and an immature male Brachypelma smithi pair. How do you know when female tarantulas are mature? Are there any signs or do you have to wait for them to tap the ground looking for a mate?

 

[Ceratogyrus]

Check to see if the spermathecae are darkened on the next moult