How can you tell if a lump in the skin is benign or malignant?

Unfortunately you cannot tell whether a growth/tumour is malignant just by looking at it. In order to be certain the growth needs to be surgically removed and sent to a pathologist for analysis. Alternatively, a fine needle aspirate (FNA), a sample taken with a needle and syringe, can be performed on the tumour by your vet. This sample is then examined under the microscope in the clinic. Often the vet can see from the sample if the growth is benign or malignant, which can help making a decision whether or not to surgically remove the tumour. It is important to note, however, that sometimes FNA samples are not diagnostic. This is always applicable to mammary tumours, hence why surgery is always recommended in these cases.