How can I tell which package updates will require a reboot or service restart?

Any package which installs/upgrades a running daemon will need a restart for that daemon. This is usually a very quick process, just a blip in service availability.

The only packages which need a reboot are kernel updates. You cannot upgrade a kernel without a reboot.

glibc updates may need you to restart a lot of services, or a reboot. However, given that most Unix services are still short lived due to the forking model, glibc updates are handled automatically.

The best way to know what is going to cause an outage is to actually apply them to a test machine and see what breaks. Testing is always a good idea.