The U.S.A. Bill of Rights is a good example for study. We know exactly in what time frame and context it was written. At the top, we have the long S in
Congreſs of the United States
and the title is boldface italics (so to speak). This long-S does have a cross stroke, but it's not clear from the image whether that's a background line or actually is a cross stroke.
If you scan down through the document, you'll see many examples of double f, and near by examples of long-S-short-s. For example,
be subject for the same offence (Article 7 line 2)
is directly above
without due proceſs of law. (Article 7 line 3)
In this specific document, there is a difference between the two letters. Look at the direction of the lower part of the letter (clockwise vs. counter-clockwise). For a single isolated lower-case F, the descender is always formed counter-clockwise. For a long S, the descender is always formed clockwise. However, in the case of double-F, the first F's descender is clockwise, and the second F's descender is counter-clockwise.
I have never seen that distinction "officially" explained, but I have noticed it in other early 19th century American (USA) handwriting.
I think I have only seen the long-S as the first letter of a double-S combination. But I've only looked at USA handwriting, 1680-1880. Hopefully these minor distinctions will help you recognize the difference more easily!