Only real way to test and see what she is is to breed her to a black or blue male only - if there is a patched tabby born (or tortie) or red boy you know mom has to be the silver patched tabby genetically even if she's not showing the red!
It would be unusual if she didn't have ANY red spot at all. This is CFA's color description
SILVER PATCHED TABBY: undercoat white, ground color, including lips and chin, pale silver with classic or mackerel tabby markings of dense black with patches of red and/or cream clearly defined on both body and extremities. A blaze of red and/or cream on the face is desirable. Eye color: brilliant gold, green or hazel.
Note that the eye color can be brilliant gold. I'm now thinking she's a silver patched tabby. Keep looking for a spot of red on her somewhere