how to find out what institutions are trading a particular stock?

[jacknapier]

How would I find this out? You used to be able to get it from nasdaqtrader.com, (my click on monthly share volume) but now you have to pay for the service. I've been googling, but nothing is coming up.

I'm hoping someone knows what I'm talking about. Basically, try to read L2 and deciphering who's who.