how do I know if bulbs are dead?

I've been digging up an old flower bed in the middle of the lawn with a
view to grassing it over.
Although there are no flowers left, I find lots of white bulbs are
still in the soil. Does the fact that they are white mean that they
are still alive and active? It would be quite a lot of work picking
them all out and I would leave them where they are if I knew they're
were dead. If they'll still alive, how can I kill them?

.