Increasingly they monitor the activity using 3-D and 4-D seismic techniques (the fourth dimension being time).  I had a reference to that somewhere relating to Ghawar and I'll see if I can't dig it out.  

Unfortunately as we have discussed before (and I will again in a couple of Saturdays as I get more into horizontal well development) once you get much water into a horizontal well then it is done.  There are a couple of remedial measures you can apply (multiphase pumping and intelligent valve control) but these are very expensive to put in, and I think that IWC has only just been tried for the first time at Shaybah as Ali mentions - though it has always been part of the plan for Haradh (which will see an increase of 300,000 bd at the beginning of next year

The $280 million Haradh-3 project aims to increase production capacity at the Haradh oil field to 900,000 bbl/d by February 2006. This will involve adding a third, 300,000-bbl/d GOSP to Haradh (in addition to two other 300,000-bbl/d GOSPs, one of which was inaugurated in January 2004). Haradh also will produce significant volumes of non-associated natural gas, natural gas condensates (perhaps 170,000 bbl/d), and sulfur.

. I got the quote from the EIA country brief, in which it also mentions that Shaybah is now looking to increase production another 300,000 bd, which has not otherwise appeared on any  published plan that I have seen, but may be the South Shaybah develpment that Ali is talking about.