How do we come to know the availability of a Technician based on his current workload with respect to record assignment?

Go to Project Configuration Settings->General Settings->Project Settings->Config Settings ->Consider state categories to calculate technician load and specify the state categories needs to be considered to calculate the current ticket assigned count for a technician. We can specify multiple state categories by providing comma separated state category ids.

Note: In classic legacy UI, it is available in all the patches where as in New UI, it is available in patches starting with 5005.

Please refer online help link for the configuration - Config Setting