Hello and Thank You for using Just Answer. The best thing that you can do is to keep the lines of communication open and honest. Tell her that you found a knife in her room. Tell her that you are not angry but was just wondering why the knife was there in her room when it should be in the kitchen. Wait and see how she responds. It may be really negative but do not over react. Just explain to her that you are open to her coming to you about anything. As for her room staying clean, explain to her that you are aware that you are not her mother but would like her to follow the rules of the house when she is at your house.
As a side note, a 14 year old girl is often very moody. Any little thing can set her off. If she continues with the knife in her room, I would contact her pediatrician. She may need counseling especially if you do not know if the knife would be used on you, herself, or someone else.
Have a great day!!!