There is a lot to consider. Firstly is the mining stock an explorer or producer. If its a producer it is a little easier because you will have a pretty good idea of the mine life and the earnings from year to year on which to make a valuation on future earnings. The only variables will be price of the commodity and mining costs.
If its an explorer or a Junior it can be very difficult to put a valuation on it. Exporation stocks are priced on a number of factors, in theory thier value should increase the further along the exploration they are but this is not always true. Important things to look for in exploration companies are drilling results (grade and intersections) and resource estimates. In the US/Canada resource estimates are prepared under what is called a 43-101 report which will estimate the size of the mineral resource according to exploratory drill results.
Key things to consider when evaluating a mining stock.
What is the size of the mineral deposit, how much drilling has been done?
Is the resource proved by drilling or inferred (ie estimated)?
What are the grades?
Is it economic to mine? (in many cases the grades might not be good enough and the property wont be a viable project)
Then you have to look at things like capital costs to get into production. At a minimum this would be $50m going up to several billion for some of the larger gold/copper/base metal mines. Infrastructure is important, is there existing road/rail or will it need to be built. If it needs to be built how much will this cost, and how long will it take to be approved.
In summary is not easy, most publically traded miners end up losing most or all of thier value. However the rewards for getting onboard one of the few mining stocks that do make it, are great.