Can an employer force its employees to take mandatory pee testing?

According to my understanding of employment law....your first step is to look at the collective bargining agreement. This agreement will control the employer's rights and the employee's duties.

if there is no collective bargining agreement, then look to terms and conditions of employment policies agreed to by the employee.

If there is no collective bargining agreement and no other agreement between the employer and the employee, then the employee may be "at will" and the employer can request random pee testing and other tests that are reasonable.

For smaller employers, if they put into place a blanket policy that does not discriminate between employees and there is a legimate safety concern, yes, pee testing can be deemed legal.