Strictly speaking, marking is not really unclean: The cat knows very well where the litter box is and uses it - but her sex drive leads her to inject her urine outside the litter box and to work her way through the whole apartment.
This is how you recognize that cats are marking
In the case of pathological or uncleanliness due to stress, cats do small and large business outside the litter box and often use carpets and other textiles or places that are right next to the litter box. When cats or cats mark, it looks different: They usually do this standing up and have often made themselves known before by loud muttering or restlessly roaming around.
When marking with urine, cats can also be recognized by the fact that they wiggle their tails. Door frames and cupboard walls are popular places for marking, but marking cats also often target personal objects and - to the delight of their owner - leave marks in their clothes, shoes or in bed. At the latest now, as a cat owner, you should think about keeping the cat in peace and having it neutered for your own good.
Uncleanliness by marking: neutering usually solves the problem
Basically, it is usually better for domestic cats as well as for free dogs to be neutered. After the procedure, marking is usually a thing of the past. However, it may also be that your cat does not mark for hormonal reasons, then neutering naturally does not change the undesirable behavior.
Ten cats enjoying the sun to their heart's content
Sometimes she has just been neutered so late that she has got used to the behavior, or she uses the scent markings to contact her fellow species. In this case, ask your vet for advice or try to fight the uncleanness with Bach flowers or harmony-promoting sprays.