If you have a single mesh renderer component containing a list of materials and you need to detect the correct material (or sub-mesh) at runtime upon a mouse click, then this script is exactly what you need. Simply attach this script to any game object. Ensure that the object requiring material selection has a collider component attached

MaterialPickOnClick.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class MaterialPickOnClick : MonoBehaviour
{

    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            RaycastHit hit;
            if(Physics.Raycast(ray, out hit))
            {
                MeshRenderer meshRenderer = hit.collider.GetComponent<MeshRenderer>();
                


                if (meshRenderer != null)
                {

                    MeshFilter meshFilter = hit.collider.GetComponent<MeshFilter>();
                    Mesh mesh = meshFilter.mesh;
                    int totalSubMesh = mesh.subMeshCount;
                    Debug.Log(hit.collider.gameObject.name + " SubMeshCount " + totalSubMesh);
                    int[] subMeshesFaceTotals = new int[totalSubMesh];

                    for (int i = 0; i < totalSubMesh; i++)
                    {
                        subMeshesFaceTotals[i] = mesh.GetTriangles(i).Length / 3;
                    }

                    int hitSubMehsNumber = 0;
                    int maxVal = 0;

                    for (int i = 0; i < totalSubMesh; i++)
                    {
                        maxVal += subMeshesFaceTotals[i];
                        if(hit.triangleIndex <= maxVal - 1)
                        {
                            hitSubMehsNumber = i + 1;
                            break;
                        }
                    }
                    Debug.Log("We hit sub mesh number: " + meshRenderer.sharedMaterials[hitSubMehsNumber-1].name);

                }
            }
        }
    }
}