Once a month the short interest of each stock is "approx" updated.There really is no way of knowing the exact number of shorted shares,in relation to the stocks float IMO.
A simple example is INTC,which can be looked at for free on yahoo finance.com(click key statistic's on the left hand side)
http://finance.yahoo.com/q/ks?s=INTC
However the accuracy is not 100% by any means,but at least you can get a general idea of each stocks individual short positions:
INTC Share Statistics
Average Volume (3 month)3: 67,022,900
Average Volume (10 day)3: 92,162,800
Shares Outstanding5: 5.84B
Float: 5.82B
% Held by Insiders1: 3.19%
% Held by Institutions1: 60.80%
Shares Short (as of 25-Sep-07)3: 72.27M
Short Ratio (as of 25-Sep-07)3: 1.2
Short % of Float (as of 25-Sep-07)3: 1.20%
Shares Short (prior month)3: 74.38M
Stephen [img]/images/graemlins/cool.gif[/img]