You can check it by using ESXTOP:
1. Run esxtop
2. Press "m" to enter on memory statistics
3. Press "f" to change the fields displayed
4. Press "f" again to show NUMA related fields
If numa is enables, you will see values (like, 0, 1, 2, etc for each node) on the NHN column, like this image:
(In this picture there is a VM running on 2 NUMA nodes).
If not, you will see the "-" on the NHN column:
Hope this helps!
NOTE (edited on Sep 17th, 2013):
There is also a command to check directly if there are and how many are the NUMA nodes:
esxcli hardware memory get | grep NUMA