That's super annoying and cost cutting is to blame. One way to tell is if you can see the backside of a Leo and peak inside, a tell-tale is if it has a nude color inside. Typically today lined Leos will be nude color. That won't tell you if it's shelf lined or fully lined though. Nowadays its going to be rare to find fully front-lined leotards. Usually it's reserved for white or really light colors. Usually the description should tell you if it's shelf or fully-lined. Alternatively, some custom order Leos let you add a lining. I have also noticed that newer leotards definition of a shelf lining is a bit of a joke, it doesn't have any supportive elastic on the bottom to 'hold' you in.
Check this Theatricals leo description for example or Lulli leo description. As for Bloch and Capezio, it looks like some have it, and some don't.
Really like something but are hesitant to buy it? Contact the seller/manufacturer and ask them for product info!