Here's a simple method which seems to work pretty well:
Ask yourself if you are more similar to your comparative type (shared first function) or your look alike type (shared second function). If you said the comparative, your subtype is probably the first function. If you said the look-alike your subtype is probably the second function.
For example:
If you are an INTj (TiNe), your comparative is ISTj (TiSe) and your look-alike is INFj (FiNe). If you are more like the ISTj, your subtype is most likely Ti (shared Ti first function). If you are more like the INFj, your subtype is probably Ne (shared Ne second function).
If you are ESFp (SeFi), if you more resemble your comparative, ESTp (SeTi) you're probably Se subtype. If you more resemble your look-alike ENFp (NeFi), you're more likely the Fi subtype.
I think subtypes are on a continuum. Some don't have a clear subtype. Some have a slight inclination towards one subtype, others have a very clear subtype.
Or you could also read the
subtype descriptions here, and see which one you relate to more. Scroll to the bottom of the page.
Hope that helps!