How do I find out if PAE is enabled?

What's the command to find out if PAE is enabled ?

Firstly, your CPU needs to support it:

[madhatta@www ~]$ grep -i pae /proc/cpuinfo 
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc pni monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc pni monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm
                                     ^^^

Secondly, your kernel needs to support it. For CentOS, this is built into the kernel version string:

[madhatta@www ~]$ uname -r
2.6.18-194.32.1.el5PAE

Your distro may have different policies, so if you can tell us what you're running, we may be able to be more help.

Check your kernel options:

egrep "HIGHMEM|X86_PAE" /boot/config-`uname -r`