I am supposed to trim my beard without a mirror and I use my palm to make sure the beard is even. How to recognize when I trim my sideburns?
Try using mirror alternatives:
- Put your phone in the camera mode where it is like a reflective surface. This way you can see yourself without a mirror.
Picture taken from Wikihow.com, Demonstrates Silver paper mirror.
- Make a mirror. Take some aluminum foil, silver paper or other foil and adhere it to a hard backing. Put it in a decorative frame and make sure you don't wrinkle it and that works pretty well to.
This may not be possible, but have others trim your beard for you.
Using a bowl of dark water also helps Darken the water with dyes or use a dark bowl. This way the water becomes reflective.
Have a rigid shaving routine and use your fingers to feel for missed hairs.
When trying to solve a task without visual aid, I often find it useful to investigate which routines blind people have come up with.
I found this instructional video of a blind man showing how he shaves and I guess it's about as good as you can get with practice.
I believe the question is not "how to shave without a mirror" but rather "how to shave without any reflecting surface or someone else helping you". If that's the case, you have to rely on sensory input other than visual.
You've already said you use your palm to make sure the beard is even. I can't tell if that means you hold your hand against your neck and shave along the edge or if you shave and then touch it to check and touch-up until it's even. However, either idea should work.
For me, I partially rely on muscle memory to get it as close as possible the first time. I will also close my eyes and focus on where the blade is on my face. For sideburns, I use an electric razor so I just attach a guide to get the length I want and, again, try to sense where the blade is to get both sides even.
By definition, it's impossible to tell if it looks right without a way to visually confirm. You should be able to get pretty close by carefully shaving and checking by touch after the fact.