There are a few key techniques that I use at low level to ensure that I am not caught out by poor weather and that I have a plan to action in the worst case should I find myself out of VMC limits close to granite.
The only true way to judge the visibility at low level in any given location is to compare the current vis to known distances on a map. It takes a considerable amount of experience to eyeball the weather and make sound judgement on the actual conditions, especially when you include factors such as 'press-on-itis' i.e convincing yourself the weather is fit to achieve the task. I think that a paper map is very useful in this situation and military pilots always fly with one at low level, although they may not always use it (Good weather and moving map). Learn at your cruise speeds how far on a map 5km is, it might be the span of a finger or half the length of your pen etc. When you fly over a known feature, use this measurement to find a feature 5km away and look for it. Do this on each of your trips and you will soon find yourself estimating ranges with increased accuracy.
The keys to not being caught out at low level in my opinion and from experience of flying fast jets at low level are the following (based on having an ability to instrument fly)-
1) Brief the route:
What is the forecast weather? What is the terrain doing over the route? Rising terrain increases chances of being forced in to cloud. Go through each leg and look at the what ifs? 'Poor weather on the nose on leg 2, low ground to the east, likely to have better conditions, however it is hard walled with controlled airspace.' Use this technique around you whole route, literally sit and talk it through with your passenger of fellow pilot. Think about your worst case options so that when you get there and the radios are busy you have already considered the likely choke points and pitfalls. Time spent on recce is seldom wasted!
2) Know the safety altitude and if it is above the base of controlled airspace for each leg:
The ideal options in poor weather should be to step around, turn around and finally if caught out accept it and carry out a low level abort to a safe altitude. Once you make the decision that the weather is unfit and you cannot turn around or step around the weather then level the wings transfer to instruments, forget EVERYTHING else and get yourself straight and level above safety altitude. I would much rather have to apologies on the telephone to an ATC agency for an infringement then to find myself in the side of a hill due to not wanting to abort into controlled airspace, neither situation ideal but one is certainly better than the other.
There are many other considerations to think about given the conditions of the day and what you are trying to do but always at the back of your mind must be the key point that: If you inadvertently find yourself in cloud below safety altitude your primary focus MUST be to get above safety altitude at all costs.
I offer my humble opinion as somebody with limited GA experience but enough low level experience through military flying to hopefully offer some food for thought. Like with most forms of flying it is a constant work cycle and current weather should be featuring at every turn of the wheel.
Cheers,
Mr Vice.