Calculating your own film speed is not an easy task and bracketing as has been suggested here is certainly not the way to go. You are bound to get one exposure correct in the bracket but depending on the contrast range in the subject you are not certain to give the film the correct development. The factors that govern the quality of information on the negative are both exposure, which determines how much information you get in the shadows, and development which controls contrast and the density of the highlights. hence the old addage "expose for the shadows and develop for the highlights"

The Zone System is one way to go but it can get complicated and is not necessarily the best way for 35mm or roll film. The subject of personal film speed is quite involved and too long to explain in simple terms here and I ask your indulgence, for I would suggest that you have a look at my book, Creative Black and White Photography where I have given a detailed, but relatively simple description of working out your own film speed and how to deal with extreme high or low contrast situations. Please forgive the plug for the book for I don't believe that members of photographic forums should use them for personal publicity.