Unfortunately, as Desley has pointed out, there is no way of telling which cats will develop FIP, as FIP is a mutation of the coronavirus, which many cats carry with no ill effects anyway. Those that do show ill effects following exposure to the coronavirus may have mild diarrhea, which is often just put down to a tummy upset. It is is only in a very small percentage of cats where the coronavirus mutates into FIP. The coronavirus is common in multi cat households, like breeding catteries. It sometimes happens that when a kitten carrying the coronavirus goes to its new home from a breeding cattery, the stress of the move etc can cause the virus to mutate into FIP. You can test for the coronavirus, but that still won't tell you whether a cat or kitten will develop FIP.
Very sad to hear about Monty.
