I can tell when the bobbin is empty on my machine because the feel of the fabric running through is different. I notice it almost instantly when it is empty anymore.
For one thing not just the feel of the fabric but the puffiness stays up when the bobbin is empty another way I can tell but I can tell mostly by the feel of the fabric when it happens.
I also keep a case full of wound bobbins so I can change without having to sit there in fill one up.