But I think I can provide some useful information.  

 
Orchard Towers is no longer a monger heaven. There may be a few ladyboy bars still open in the area. But the clubs that made the "Four Floors of Whores" famous are all closed.  

 
Geylangs remain a viable option. Operating daytime to late night, you can walk in and take a provider to a room.  Expect no kissing, CBJ (may find BBJ in some), CFS for a moderate amount.   There is no risk of LE action. The Geylang houses have been tolerated (and used by) LE for years.  

 
Finally, you may have good luck at Spa locations. You will need to research off TER to find current locations, rules and prices.  

 
There will be no providers under 18, as these establishments risk serious legal issues if they do not verify age. Still, that is not a guarantee. The simple rule is: if she looks too young, leave. Don't just pick another at the same location. Leave!

Have some strange rules, customs and laws.

Would be to just go ask a police officer at a police station. Given prostitution is legal and the red-light district is hardly something that is hidden from the public. The legal brothels are regulated by the government so I would think the police or some other governmental officials. But you might be averse to the direct approach. One of the guys that worked for the same company I did once referred to "the naughty side of town" so not like people don't openly speak about things like that.

 
Or you could just ask to see the license the brother should have, ask to see the health card for the girl (and perhaps check when the last exam was if that is listed). Of course none of that is likely to be in English so.... Also, seems that the brothels will all have the house numbers in large red numbers. Non-brothels will not have any red.