They should be able to test it without the passcode. The main screen shows battery and they can test the drain without being logged in. Some places will let you hold the SIM card which should prevent access to any data.
Since you are going to an off-brand store, you'll need to take a careful look at the edges when they are done. Make sure the glass is squarely inside the frame without an edge sticking up. Backside too.
Then you should test it by using a battery draining app like Pokemon Go for 10-30 minutes and see if the battery is holding steady, and not dropping by 25%.