VMC
Its only partly about the weather.
Weather radar, METARs, and TAFs are without doubt the best sources for buidling a picture of the weather, and build a picture you should. It isnt of course just about the weather along your track, but the whats happening to the windward of your track that will give you a feel of how things might develop. Equally important is an assessment of your escape routes along your track - where do you go to get quickly on the ground if necessary and how might the terrain impact on your decision.
I find it interesting how often pilots plan for trouble. For example consider a typical trip from the south of England to Scotland - there are plenty of possible routes but why would you select a route over high ground if the weather is marginal - add on the extra miles and route up the coast.
and its also a lot about your piloting ability.
So many pilots seem very uncomfortable flying low level - is it stories of the dangers of scud running or other reasons? It takes time to be comfortable flying low level but once you are comfortable in reality it is very rare you cant stay under the weather in the UK. I can think of very few trips that I have wanted to do VFR that werent possible. Yes, that may well mean a circuitous route and there may be occasions when it clearly isnt possible or wise.
I would of course recommend time spent with an instructor or another pilot who is comfortable flying in these sort of conditions, the usual health warnings apply.
As to second guessing the professional forecasts I am less certain they are as consistantly accurate as some would have you believe. I can think of one pilot I know well who has a better than 90% record of predicting what the actual weather will be in those circumstances when he thinks it will be different from the met office forecast. More often that not it is better, but to be fair I can think of a few occasions when we have rightly predicted it would be worse.
Of course in these days of computerised weather forecasts local knowledge is particularly useful. There has been more than one occasion my flying partner(s) have been anxious to depart based on good TAFs - On a few of those I have been much more cautious and asked them to be certain that they really dont mind a train journey on our return to collect the car. Sure enough the fog has rolled in and all movements for the rest of the day have stopped.