I too have a passion for old film. Anyone can get an image on new film. The challenge of an image on old film is fun. That said, there are set backs. I found a partailly used 4x5 plus x film box that expired in 1973 at a camera show last year for $2 and it tested as ISO 3 as it is now. So I now keep it around to use as a pan masking film.
I test them if they are 120 or 4x5 in the camera using step wedge in the flim holder, or over the film gate, and expose as though ISO 2 while aimed at a neutral grey test card. 35mm I test by bracketing exposure on a grey card and cli testing; my methods may use up most of your 20 exposure roll; most of the time I use my method when I have a whole lot of film of one batch that I want to calibrate.
I then develop the film per normal times plus 30% as a start if they are really old. I try to have the developer warmer, and then use a kodak dataguide development dial to figure the right time that is equal to a 30% push at 68F. I do this in the hope that with the warmer developer, the less time there is for fog to develop. If the test for base iso works, then shows any edge marking on the second sheet I use as well as the subject content can be used to tweak the development time.
After development, I throw the step wedge test shots under a densitometer that I am lucky was gifted to me to figure which step gives me a normal Neutral. Visually you can compare them to a regular neutral grey neg density by sticking half of each in a negative carrier and then project them on the base board if you don't have a densitometer. A step wedge either has 1/2 stop or 1/3 stop steps, so that lets you figure out by counting steps how far off of ISO2 your film really is. If the NG happens after say 7 half stop steps then you still have a
film with iso20 behaviour.