Examine the primary bud, which is the central cylinder in the middle of the fruiting node. Make cross-sectional cuts through the bud about one-third to halfway up the height of the bud, and look to see if it's green or brownish-black. Check about a dozen buds. If injury is confirmed by finding brownish-black coloring, an in-depth examination of about 100 buds needs to be done.
Examine the cambium tissue of the canes. This is the thin layer of tissue found between the bark and the wood. It is bright green if healthy, and light-brown to black if the vine is injured or dead.
Vines that have suffered moderate injury from the cold need to have injured tissue removed. Vines that have suffered severe injury, in the 60- to 100-percent range, should be left alone. New buds may start growing late in the spring and there may be new sucker growth. New suckers can be used to grow new trunks.