I hope someone might have insight (or better yet a textual reference) for a cultural convention.
In Viśvanātha Cakravartin's commentary (ca. late 17th century) on the Bhāgavata Purāṇa 10.9.7 (uttārya gopī...), where Yaśodā finds the curd-pot that Kṛṣṇa had broken, he adds a comment that she touched the tip of her nose with her left index finger (vāmatarjjanyā nāsāgraṃ spṛṣṭveti jñeyam).