Determine if a substructure is present in a chemical structure using SMILES string (substructure fingerprint)
Mathematica, 31 bytes
MoleculeContainsQ@@Molecule/@#&amp;

Takes input as a list of 2 strings (the source and the pattern molecules).
As you could guess, this checks if the first molecule (parsed via Molecule) contains the second one by using the MoleculeContainsQ function.
This doesn't seem to work in the online interpreter on TIO; I'm not sure what I am doing wrong. It works on my local machine, though. Of course, this is not using an external library: it's completely built-in functionality!