I use a combination of the river level gauge near where I live, the colour of the water and the colour of my car. Today the river was 0.6 metres above normal and falling - OK, the colour was very thick - not OK, and my car was white - not OK as it is red ie heavy frost. So I went and blanked elsewhere, just nice to put a float through. Since then 8 hours heavy rain have fouled any chance of fishing it tomorrow.
Interestingly on the stretch RoachMad mentions I have fished when the mill a few miles upsteam has been working (mid week) and had the level rising and falling all day but not due to rain, and caught all day which means his level would be hard to read. Similarly slightly downstream I've fished when the mill has shut its gates and the level has risen fast and the roach have fed very well as the river has steadied.