It is common for a ferret to develop lumps as they grow older. Unfortunately, just as common is for those lumps to be cancerous in a ferret.
Any lumps you may discover on your ferret need to be looked at by a veterinarian to determine what the lump is and possibly remove the lump.
A veterinarian will examine the lump by manipulating it to determine the shape of the lump and to determine if the lump is connected to surrounding tissue.
A veterinarian may insert a needle into the ferret's lump and remove some fluid for further evaluation. Inserting a needle into the ferret's lump has minimal pain involved and this procedure is usually done with the ferret awake.
If the veterinarian determines that the lump needs to be removed from the ferret, it will be done under anesthesia and the lump will be sent to a pathologist to determine exactly what it is, and if any follow up treatment for the ferret is needed.