You know you're in trouble when you find your self slowing down and descending to stay clear of the soup.
Set yourself a minimum for height and speed, as soon as you see yourself descending and/or slowing down below these limits - LAND (or turn around if applicable).
That could be 1000ft AGL, 80kts or 300ft AGL 50kts or whatever, obviously depending on what you are flying over, and at what time of the day.
As with all "personal minimums", the trick is to actually stick to them when it happens, even if you are just 3 miles from your destination. Never change the rules in flight.