Firstly, if you have been made aware that one of your staff may be impaired in the workplace under your duty of care you are compelled to act and address this potential safety risk. You can have the individual tested but without a compliant D and A policy in place you will be unable to dismiss the employee if the result returned is positive. This is only recommended when you have a compliant Drug and Alcohol policy and procedure in place and all your staff are aware of the policy and have been given adequate time to read and understand the policy.