It depends….Most SANs have the ability to monitor the IOPs of each volume. So the volume that is mapped to your ESX server can easily be monitored as well as any other server that is mapped to a volume. But it DEPENDS on the SAN you are using.
There are other various tools that can be used for their perspective operation systems. I googled “AIX iops” and got a bunch.
For Windows, I like the IOverifyMFC. I dont remember where I got it, but it works great. There are a bunch of parameters that you can set to emulate your environment.
For Linux and ESX, I like fio. Its on freshmeat.net.
I hope that helps.