Hi,
Here is a list of the indicators of a Walk In apartment:
1. You can hear fake female moaning
2. You can hear Man grunts
3. There is a cylindrical pink/red neon light hanging above the door
4. There are multi-coloured cut out signs in Chinese or English outside the door
5. There is a sign on the door that says "ring bell" or "please wait"
6. There are pictures of naked or semi naked women on the door
7. There is a small camera above the door
8. There is a happy or disappointed bloke walking out of the apartment
9. The door opens with a smiley woman who is very happy to see you
10. The door number and address matches the one you saw in the DB
11. There are guys waiting outside of the door
12. You can see one guy or guys who is walking back and forth all the time
So if you see one of these indicator there is a 50% probability, with two indicators it is 70 % and with three or more 99% (except for the combination of 2, 11 and 12 because it could be a public toilet
)