How Can I Know That My Job Is Reliable?

There are various methods of knowing about the reliability of your job. Firstly you can know it from the size of the company you are working in. Bigger the company more reliable is your job. It has been my own experience and an admitted fact all over the world that the established companies do not let go their employees so easily. This is one of the reasons for their establishment. They consider it a loss if their employee leaves the job and switches over to some other company as he will take his experience and learning with him to another company.  In same way, they do not fire their employees until and unless they feel it unavoidable.
You can also know about reliability of you job by knowing about the employees that have been fired by the HR. If the ratio of employees fired by the HR is high then you must not take you job as reliable and you must start looking for a new one.
Similarly, the previous employees can guide you a lot about the reliability of your job. If the ratio of resignations is high and if people working in the company are looking for some opportunity outside the company, even then your job is not secure.