Does anyone know?
 

How do you know when your bobbin is going to run out of thread before you sew 'a mile' without any?
My sewing machine does not even whisper a warning to me!
I know there are new sewing machines that do, but I love the one I have.

I can tell when either my bobbin starts running low when my stitches start either getting longer or with the bobbin the thread will start bunching or the needle seems to catch when it goes down to pick up the bobbin thread.

I know when I see it isn't stitching any more. I have two straight stitch only machines, I love them and use one or the other all the time. A machine that tells you would cost more than these. Wouldn't be worth the cost to me.

I also know when I see no stitches. Yes I have a machine that will beep at me but it seems like it beeps at me too far in advance, so I ignore it, then I finally turned it off. So that feature doesn't do anything for me.

One of my machines has the low bobbin indicator. My straight stitch only does not. It seems to sound a little different when the bobbin runs out.

Mine has a slightly different sound as I am stitching when the bobbin is very low. I don't always pay that much attention and notice this long upper thread hanging around. That is a big clue for me to rewind.

Mine's supposed to tell me, but it doesn't always. The only way to tell is looking at the stitches, and seeing none.

Wind two bobbins. Put one on the top and the other in the bobbin. The machine should use thread from top and bottom at about the same rate. You can keep an eye on the one on the top. This is only going to work if you use the same thread in the top & bobbin!

This is me as well. If I'm doing something 'quick & dirty', the dreaded mending or such, I might notice the change in sound but otherwise, not so much. I've usually got the TV going and get in 'the zone' if I'm doing lots and only notice when I get to the end of whatever piece I'm working on.

I compare one bobbin to another. Depending on what I'm working on. I check the amount of thread in the bobbin case and compare it to a full bobbin. Also do a couple back stitches depending on where I'm at in the project, stop take the bobbin out and look at it. Then move on to continue or replace.