I downloaded the DMinder app yesterday. It can calculate your likely vitamin D levels, based on the supplements you’re taking, food, age, body weight and the parameters you put in around sun exposure... skin colour, location, altitude, clothing. You can set a target level and it tells you how far you are off your goal. You can also set a stopwatch for time out in the sun, and it extrapolates from sun angle & latitude what you received vit D dose is. Other good stuff it tells you the dates of the year when the sun is at an angle high enough for the skin to produce vit D (31.5 deg). Not affiliated in any way besides being a happy new user. Currently free during pandemic, I think the premium upgrade is £1.99 usually.
You can also get a vitamin D mail order test for an NHS laboratory, Sandwell in the W Midlands, for a modest charge (£29 ish). You are sent a kit including mounted blotting paper and finger pricker. Send it back and get results in a few days.