How do I truly know if certain roses died over the winter?

I don't know what zone you are in, but if you are not seeing any green suckers or leaf buds by know in zone 5 or warmer - they are dead. You can tell if a cane is dead by just taking your nail and scratching at the brown of the stem,if the underlying area is green it is alive, brown underneath means dead or dying.

You also might want to see if the roots have been eaten. Voles love roses and will spend the winter nesting by the roots and eating them until the bush will just fall over.

For the roses starting to come back - sprinkle a little epsom salts by the base of the shrub. It will encourage new growth.