How can I tell if a machine has PAE?

Citing https://help.ubuntu.com/community/EnablingPAE:

To check if your processor supports PAE, try

grep --color=always -i PAE /proc/cpuinfo

If it outputs something, you have PAE support. Otherwise, the output will be empty.

Another option (which uses a GUI) involves using Hardinfo (System Profiler and Benchmark.

• Under devices, select Processor.
• From here, you can see the processor's capabilities (along with their simple descriptions).
• If PAE is not listed, your processor does not support it.

• As you can tell, the processor in this example does not.

• and this one does.

From the terminal, simply type the following.

cat /proc/cpuinfo

Scroll down and check the flags. PAE will be listed in the flags if supported.