To get the internal ID of each items in the system.
1. Navigate to Lists > Accounting > Items.
2. You can select - All - or specific item type.
3. Then click Customize View button.
4. Kindly add "Internal ID" on the Results tab so that once you export this results, you will be able to get the Internal ID as well which is very important if you will use CSV Import feature to update your items.
Other ways would be to check it on the item record's website URL. This is only applicable if you want to get a specific item's Internal ID:
1. Open up one item record and check its website URL: