Dumb question about yeast starters

So I have been reading about yeast harvesting and yeast starters on here and have learned quite a bit. One thing that I am still not sure on is how you know if your yeast took off in your initial starter or when you are ready to increase the size of your starter?

There is no airlock so no bubbles will be evident.

Do you take a gravity reading to determine if your harvested yeast were viable and took off?

Also when adding more wort and increasing the size of your starter do you stick it in the fridge to let the yeast fall out of suspension, decant liquid, let warm to room temp, & add new wort?

So, my process is to let the starter ferment and give it a day to settle out. You will see a layer of yeast on the bottom of the flask. There is still a lot of yeast in solution but I decant anyway and add fresh wort to the layer of yeast on the bottom.
Also, you know that the yeast is viable because when you swirl or agitate the beer it will degass the co2 in solution. You may or may not see krausen form on the top but will see co2 bubbles as the yeast ferments the wort.
Hope this helps.
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well it should be obvious if your yeast took off. there will be a thick layer of stuff at the bottom of your starter. typically 2-3 days after starting. as far as increasing the size of the starter your method sounds about right.

I use a stirplate. I check the movement of the liquid (at the same plate speed) to try to gauge where yeast is in the cylce. At first pitch it will be relatively fast, then it will slow down (i.e., the slurry thickens up), then it will speed back up again when it's about "done." Generally takes anywhere from a few hours to a day or two. Not sure if this is really accurate or not, but it seems to work for me. I've never bumped up a starter (biggest I've done was 2L, which is the size of my flask), so I'm not sure about the procedure for going bigger...