It can take a few days after a frost before you can accurately answer that question. With a light frost of 0 C to -2 C, the risk of crop damage is minimal. With a heavy frost that kills off the cotyledons, it takes a few days before new leaves will start emerging from the growing point between the cotyledons. If no growth occurs within this time, the plant is likely dead. Also, if the stem is pinched off or the plant flops over, the plant will likely die. The pinched off or broken stem cannot provide nutrients to the growing point.
Early season frost can thin a stand, but in most cases a sufficient number of canola seedlings will survive. Most often reseeding a frosted field will fail to improve returns.