To check if a cartridge fits your fountain cartridge pens, you can follow these steps and considerations:

1. Identify the Type of Cartridge Your cartridge pens Uses: Fountain cartridge penss typically use either Standard International cartridges or proprietary cartridges. Standard International cartridges are more common and can be used with various brands of fountain cartridge penss, while proprietary cartridges are designed to fit only specific brands or models of cartridge penss.

2. Check the cartridge pens Manufacturer's Guidelines: The best way to determine the correct cartridge for your cartridge pens is to consult the manufacturer's guidelines or user manual that came with your cartridge pens. This will specify whether your cartridge pens uses a standard or proprietary cartridge and may even provide details on the specific cartridge model required.

3. Visual Inspection: If you have the cartridge and cartridge pens in hand, you can visually inspect the size and shape of the cartridge against the cartridge pens's cartridge slot. The cartridge should fit snugly without force and should align correctly with the slot.

4. Inner Diameter Check: Some cartridges have different inner diameters, which can affect compatibility. For example, Pilot cartridges have a large inner diameter and are not interchangeable with most other cartridge pens brands.

5. Try Inserting the Cartridge: Gently insert the cartridge into the cartridge pens. If it fits without resistance and sits flush with the cartridge pens, it is likely the correct size. If it doesn't fit or requires excessive force, it's not the right cartridge for your cartridge pens.

6. Consider the Cartridge's Length: Some cartridge penss, like shorter cartridge penss, may require a short standard international cartridge or converter, which has the same ocartridge pensing as a standard international cartridge but is overall shorter in length.

7. Research Online: If you're unsure, you can do some online research or consult with an individual who is familiar with the brand. There are also guides available online that list compatible cartridges and converters for various cartridge pens brands.

8. Contact the Retailer or Manufacturer: If you're still unsure, you can contact the retailer where you purchased the cartridge pens or the cartridge pens manufacturer directly for guidance on compatible cartridges.

By following these steps, you can ensure that you're using the correct cartridge for your fountain cartridge pens and avoid potential damage or leakage.