Since all brake lines are not see through, the only way to determine if your brake needs a bleed is by feeling the brakes. If they are "squishy", then you need a brake bleed.
I'm of the opinion that sight would be more effective to determine if there are air bubbles in your system.
Are there clear hyrdraulic brake lines available for sale?
If they are not available, is there a techincal reason why they are not feasible?
Here's some examples:
The pressures necessary in any disk brake setup are extremely high, many in excess of 2000psi [SRAM 4234 Rev A(2012), p4] so require a spirally-wound fiber core reinforcement to function. Because this fiber reinforcement core can never be clear, even if the fibers were themselves clear such as glass (which is a common fiber used) you will not find any that are in fact clear all the way through.
The Shimano clear disk caliper linked is designed for demonstration purposes only, in order to show how it is much more common for air bubbles to be present within the caliper than within the lines. The demo shows an old and new design to demonstrate improved bleed action.
With the answer out of the way, your real concern is extracting all air from the system:
First, there is almost zero chance of having bubbles stuck in just the line if you follow any standard bleeding procedure as the lines are smooth and often even teflon-lined, so they are the slipperiest part of the system.
Second, the best method I've found that ensures all air is extracted from a problematic system is by first bleeding normally to purge old fluid, then adding a length of hose from the bleed nipple to the open reservoir (a friend here can make all the difference). Then simply pump lots of fluid through it at various rates and speeds to give you the best chance of dislodging any bubbles stuck within the caliper due to surface tension. The return tube allows you to pump much more fluid through the system than you would be able to do with the standard bleed process, since you are re-using the fresh fluid in the system automatically. To help with this bleed-fluid-return-tube process, you may find a few wraps of plumbers teflon pipe tape around the threads of the bleed nipple help to ensure no air ingress through the bleed nipple threads occur.
I have some very old mid-2000s design Hope calipers that would routinely have difficult-to-purge latent air bubbles in them even after bleeding several ounces of fluid through them. After switching to this procedure so that I could run pints of fluid through them without wasting that much actual fluid, even those "problematic" calipers have been sponge-free every time.
Good luck !
Your underlying question here is "how to tell if there is air in the lines?" and transparent brake hose would probably work well for that, if it can hold the pressure.
There are water-traps for air-based systems - I wonder if there is such a thing as an air-trap for hydraulic systems. It would have to be at the highest point in your hose, or integrated with the lever/master cylinder.
For various reasons, my hydraulic brake hose runs over a curve, so the lever is not the highest point in the line, and air likes to pool in the top of this curve.
A quick web search does not return any useful items sadly, but not finding does not prove non-existence.
A third option could be a two-hose brake, where fluid cycles down to the caliper from the master cylinder, but the reverse flow takes a separate hose, theoretically flushing air bubbles back into the unpressurised reservoir where they rise out, and the master cylinder draws from the bottom of this storage bottle.
