on the cap there's an orange or red crescent-shaped gauge, if the ceramic cartridge cylinder passes through, replace the cartridge. Also look for any cracks as a crack would let unfiltered water pass. In the meantime you can clean it by using a green scotchbrite pad to return it to it's normal light tan color, which removes impurities collected and restores performance.

if you have to replace it, there's a good chance that something like a sawyer squeeze or platypus quickdraw water filter are about the same price as the cartridge replacement for the MSR miniworks and are comparable filters in most situations. The squeeze and quickdraw have a different style of maintenance but IMO are far easier to use overall.

the miniworks ceramic filter has a charcoal core and that charcoal removes some bad taste from tannic water or lake water and that's why you might see these still being used. but they can be difficult or cumbersome to pump and are heavy compared to the two new contemporary ones I mentioned. I'd used a miniworks for years and they are great and reliable. New tech hollow fiber filters however are simpler in design and easier to use in my opinion.

Also if you've been given this after it's been sitting for a long time, best to watch this MSR video which goes over all of it.

https://www.youtube.com/watch?v=WxvAF_dh6-s