How to distinguish a pair of non-isomers vs. a pair of isomers:

An isomer is a molecule with the same molecular formula but different structural or spatial arrangement of atoms, this variation can lead to difference lead to a difference in physical or chemical properties.

Two categories of isomers:

Constitutional Isomers: same empirical formula but different connectivity. This can be molecules with different functional groups (functional: cyclohexane & hex-1-ene) and/or bonding patterns (positional: hex-1-ene & hex-2-ene) or hydrocarbon branching (skeletal: cyclohexane & 1-ethyl-2-methylcyclopropane).

Stereoisomers: same connectivity but different arrangement of their atoms in space. They are conformational, geometric, enantiomers, and diastereomers.

Conformational - Stereoisomers produced by rotation about a sigma bond.

Geometric – These isomers differ in the spatial position around a bond with restricted rotation (e.g. a double bond or across a ring system).

Enantiomers – Non-superimposable and exact mirror images. They are nearly identical in properties, but have opposite plane-polarized light and different properties in chiral environments. All stereocenters are exactly reversed when comparing enantiomers.

Diastereomers – Non-superimposable and not mirror images. Molecules that are

diastereomers must have multiple chiral centers and will have completely different properties. They will be no more similar than any two constitutional isomers and will have different boiling or melting points, solubility, reactivity, etc.

Further information about isomers can be found in master organic chemistry website.