Most contracts give the client some control over "how it get's done". The contractor is being hired to perform a specific task, if I as the client have some stipulations then that is negotiated at the time the contract is signed.
Who provides and who decides the materials used? Who decides the hours to be worked? Can the work be done offsite or on?
All of these can be dictated by the client or left up to the worker and can have various degrees of bearing on whether they are employer/employee of client/contractor.
In most countries there is usually at least 5 or 6 primary criteria (often with several sub-criteria) for determining employee vs. contractor and the weights given to each are generally decided case by case.