How to programmatically identify a chemical compound in my program?
Either way, you need to define what it means for a chemical to be 'in' your database. Do you care about the 3D coordinates (given that you have XYZ) or just the atom types and connectivity?

Short answer. use a cheminformatics library that provides structure matching or can encode your molecules (query and targets) as canonical SMILES, InChI, etc.

See Is there a way to use free software to convert SMILES strings to structures?

Long answer. Use a series of filters (or questions):


Are there any molecues in my database with the same number of atoms?
Do any of these have the same atom types?
Finally, are these atoms connected to the same neighbours?


The third one is a little trickier, but is still a quick way to determine isomorphism. If you  need true structure matching, see the short answer! If you need 3D matching, then again it's going to be much quicker to use a library method.