How can I check the temperature of all CPU cores without using lm-sensors?

It might be that your CPU doesn't have individual temperature sensors for each core. Since the cores are so close and covered by thermal paste and a heat sink, their temperatures usually average out unless there's something very unusual happening. One simple alternative is to check the Task Manager and look under the Performance tab to see how much each core is being utilized. That's probably your best bet for monitoring core activity without diving too deep into sensors.