Is the job permanent? A full-time worker whom you've hired for an indefinite period of time is probably an employee, regardless of where or when he or she does the work.
How integral is the work? Someone who produces or delivers your company's product is more likely to be an employee than someone who, say, inputs information for the payroll department.
Does your contractor have other clients? Contractors should be free to work for other clients, not tethered to your company.
Does your contractor have benefits? A worker receiving benefits is probably an employee -- but someone who doesn't get benefits could be one, too.