Count the number of molts starting from its first instar on the mothers back, it goes up one after each molt. If you bought it without finding out which instar it was, from the breeder, then you will be guessing. It will be adult, 7th instar after it reaches 6 inches or more.
What i do is I make a log and collect the exsuviums and make a chart for comparison purposes. I keep a complete collection per genus as of now i have heterometrus, centruroides, pandinus, androctonus, working on hotenttotta genus now.