Ours is hard wired and when we test it it screams like an air raid siren.  It is quite old but rewired to code.  The entire neighbourhood  would be notified if we have a fire lol  We also have portable CO2 detectors because we heat with gas and have gas fireplaces too.

I test them regularly with my cooking.  It was hard wired with a battery back up.  I had to replace it when the battery chirped.  It would not reset and it was covered by a warranty so I got a new one.  

I assume the home inspectors check them.  I have noticed writeups when they were missing.  I need to call them and find out their policy 

By code, alarms need to be hard-wired to be up to code here. We have smoke and CO2 detector units.

The combo co2 and detector in my home. Hard and soft wiring work well. Remember that hard wire needs the battery for back-up power loss while battery does not need anything else. Inspectors always check this

I have wired and they are all disabled, Lunapie hated when the would chirp and go try to bury herself in the back yard! And when one would go, they would ALL go. So I have battery operated portable ones and the built-ins are in storage for if I ever sell and need to put them back.
And home inspectors have routinely tested smoke alarms to my knowledge . . . 

My inspector does not test the smoke/carbon devices. He states if they are present where they are supposed to be or not present where they are supposed to be. 

I have the 10-year battery - Kidde. 

Ours are all battery operated.

Always surprised to see how often we find these missing when tenants leave, dont know why they would need to steal them since their new rental will have one in it already as mandated by state law.

The units in our building are hard wired with a battery backup. Likely not checked as often as they should be.

As of January 2018 Maryland law requires the smoke detector to be replaced every 10 years , hardwired and the battery must be concealed and tamper proof.

In my location( Los Angeles County)  the house will not pass retrofit if they are not wired. So, yes, smoke alarms and carbon monoxide devices are always tested during the retrofit inspection. 

No inspectors don't typically inspect the fire alarm. We have it all wired into our alarm system and it's tested once a year.

well it's like this..... we have ALL the kinds in our house.  The two that were hardwired (to code when the house was built) and additionally photoelectric in the areas we think there could be a smoldering fire (laundry room, near the furnace) and ionization in all the bedrooms and dining room.  We currently don't have them in the kitchen, but it's open to the living area which has one of the hardwired ones.

Well I can tell you that a hardwired one did not go off when I had a small fire. That's when I doubled up on everything. Always good to have a CO2 detector.

I have a battery-operated combo Smoke/CO photoelectric with voice in all locations. Home inspectors do test and the Fire Dept is required to test before closing and provide a certificate. I prefer battery operated. The new ones have a 10-year battery life

Fred Griffin Florida Real Estate I have hard wired and I like them. Odd thing however is that even the hard wired ones need batteries. I like the multi-purpose detectors. The inspectors in our area test them but I do not know to what extent... maybe just holding the test button to see if it beeps.

I specifically work with properties in Los Angeles, but I would recommend that you check your state’s regulations. Some states require smoke detectors to be hard wired and several states have also banned ionization smoke alarms.

I like hardwired with a battery backup (because the fire can occur during a power outage).  Don't know the technology, just know where to buy them at home depot.

Inspectors and appraisers test them.

Hard wire with battery backup is ideal but battery only is much easier to install.

I have hardwired, with a battery backup, I also have both photoelectric and ionization detectors in my house. The photoelectric is in the hall, outside the bedrooms. and the ionization is throughout the house and in the bedrooms. I just swapped batteries on them about two weeks ago too.