In general if you go up somewhere high and look at both arms of the lake you should be able to get an idea what is happening. In town Belfast Terrace, Edinburgh Drive and the top of Man street are all good spots as are many spots at Fernhill. Look down at the boats near the Kelvin Heights golf course, are they facing South or West? Look down the southern arm of the lake, can you see a darker wind line further down the lake? Are there thermal type cumulus clouds sitting on the top of the Remarkables? (maybe a southerly will come through). Look over to Walter Peak, if you can see some white wave tops just behind Walter Peak this is usually a sign that the NW is happening further up the lake.