I got mine done as part of blood tests for arthritis ( the practice nurse said I should ask for it since my joint aches were particularly bad in my neck). My husband had his done as part of a general group of blood tests before starting a course of high dose steroids. His was actually lower than mine, in spite of him insisting that his levels would be ok, because he went out more, etc.
At this time of the year, you should assume that your levels are low and get out in the sun as much as you can and eat the things with vit D in them. It is hard to get all you need from your diet, so most people will be depleted, unless they can afford a winter holiday in the sun. I take a supplement now, to prevent my levels dropping again.
Sardines on wholemeal toast are very good - vitamin D, omega3, and calcium. I like them with butter, though