My test is not very scientific. It is based on my experience raising the sheep for a few years. The larger the udders, the more likely they are to have twins. She has given me twins before and she is moving way too agile to be due soon.
I have two white Merinos ewes that are definitely pregnant and due within the next month. With their heavy fleeces, it is hard to tell how far along they are. One of them is beginning to nest - staying a little away from the rest of the herd and coming for grain as soon as she sees the bucket.
We are now on lamb watch.