It is always concerning to find a lump on your pet, but in a large majority of the time, they are a benign finding and of no consequence. As I always tell my clients, I would never diagnose a mass simply by the way it feels and the same holds true even more so when I cannot see it.
The danger signs of a growth is one that grows or changes quickly or one that seems to have indistinct borders (it's hard to delineate where the mass starts compared to normal tissue). It's not to say that lumps that don't fit that criteria are safe, but that is just a guideline.
Lumps can be very innocent and be things like benign fat tumors (called lipomas) to cysts to even sometimes inflammatory reactions. On the other hand, sometimes more serious tumors can start off small and not have any indication as to their severity.
One simple test that your veterinarian can perform is called a fine needle aspirate. This is a test where they can aspirate a small sampling of the cells of the tumor by inserting a small needle into it. Most of the time, after a test like this, they will be able to let you know if it is something you need to worry about or if it needs any further intervention.
I hope this information is helpful. Please let me know if you have any further questions