Find symbol's font slot

You can, in order of decreasing convenience:

• Use unicode-math, where you can replace any math symbol with setmathfont[range=symbol]{...}. Many OpenType math fonts have variants in stylistic sets.
• Check the package documentation for a font table.
• Check whether it uses a standard LaTeX encoding. If it does, it probably has files with names likeoms*.fd and omx*.fd.
• Search the .sty file for the symbol and copy-paste the commands needed to define it. Usually this is DeclareSymbolFont, and sometimes SetSymbolFont for bold, followed by DeclareMathSymbol.
• Make font tables for all the package’s symbol fonts and look for the symbol in there.