I am using
getWMSFeatureInfo with multiple point layers.
When I click on a point on map, how to specify which layer that point belongs to?
I am not sure I understand what you mean, but you can find a docs of WMSGetFeatureInfo object of OL here. You might be interested mainly in drillDown and layers properties. That's how you can set what layers will be queried on click.
you can get your layer information from your feature. if you alert your f info when click as shown in following code, you can get what you want.
i hope it helps you...