Not really enough information.
Different ones will put forward suggestions and will be just a stab in the dark.
For one, a pot mounted with it's moving shaft connected to your inner bolt or whichever moves.(indeterminate there also)
You can wind a solenoid of magnet wire on a form that the bolt will slide through. Then build an oscillator with the coil as part of the resonating circuit. Measure the frequency with the bolt fully in the coil and measure as the bolt is moved out of the coil. The resulting frequency change will give a very accurate indication of the bolt position.
Any reason to not trust your servo?
You could add a current sensor on your servo's supply. If the bolt gets stuck the current increases as the servo stalls.
Good idea. It's door security so paranoid... I think I'll trust servos own sensors but add a current sensor. There's two servos, low bolt and high bolt. Armatures are break away wood in case of emergency.
How would I "convert" /DC/ current to voltage so I can have MCU read it thru it's ADC? it isn't end of world if it isn't implemented, there are two per door. Just to say " hey my door has electronically controlled and stall monitoring servo actuated position sensed steel bolts".
I always assume these projects are lawful - like for protecting one's own front door.
Usually door locks are made with a solenoid that has to be kept in place, unlocking the door upon power outage for safety reasons. This way you don't lock yourself in when the power is out.
Of course if this is the door to a vault, you may want it to work the other way around
Getting out is made way by yanking on marked pull chains that are connected to bolts, they snap the servo links and pull out bolt. Alarm sounds of course.
Ingress during grid fail is a problem for me... Since my thing is servo not solenoid (no solenoid does 3") it can't unlock at moment of outage. Or should it?! That could be an attempt to force entry. I think a hidden pull chain may be best answer, careful and bend back a piece of siding and route thru pull chain... Grid failures are rare though.
I'm stuck on the access control theory now. Users, credentials, and permission... Bleh