Sometimes I forget that I can use TOP to make sure that all my cores are up and running. It is very simple:
When you bring up TOP, just hit the # 1 key. Now all your cores will be seen.
Normally I use htop, or cat the proc filesystem, but I'm lazy and now this is part of muscle-memory when I bring up a system for the first time.
A few years ago I was diagnosing a problem and using no-apic/lapic/ioapic and so forth, and system felt sluggish. Using top in this way quickly showed that I had shut down all the cores except for one. Hasn't happened since.
It's just a habit now, but maybe some guys who don't use top much might use it just for this cpu core activity check.