Things to bring:

Screw driver
To switch on water supply. The taps for turning on the water supply is in your mailbox, remember to grab them before going to your flat. Loosen the top and bottom screws and use the tap to turn on the water supply. Your unit number should be labelled!

3 pin plug with light source or any type of charger

To check on all electrical sockets. 
**Find the electrical supply box in the utility box. Flip up all the switches to turn on the electricity.

Color Masking Tapes and Markers

Color masking tapes and markers are used to mark out the defective areas so that contractors performing the rectification works can easily spot the troubled areas. In addition, it is recommended to scribble notes on the description of the defects so that miscommunication can be minimized. I also recommend putting numbers on the color masking tapes so that they are easier to keep track. 

Cameras

The digital photos provide excellent documentation of the defects before and after rectification. This helps to speed up the rectification  process especially during disputes.

Measuring Tape or Ruler and Spirit Level

Measuring tape or ruler and spirit level are essential tools to determine if there are any structural deviations with the room and wall measurements and alignment. The spirit level helps to ensure that the doors and windows are properly aligned and that they are vertically upright. The spirit level also helps check walls and ceilings are not slanted. It is used to check the alignment of the doors too.

Torchlight and Stool or Ladder

A torchlight is great for exploring dark nooks and crannies. In addition, and stool or ladders are especially useful for shorties like me to inspect those high out of reach places.

Coins or Marbles

Coins or marbles give off a distinct clear crisp sound when they are knocked against the walls or floor tiles and thus are excellent candidates to check for hallow tiles or timber strips.

Hose

To check that toilet floor sloping is ok and not accumulate water in some corner

Chair/Newspaper
After a few hours of checking, you will want to sit down for a rest

Number lock and Chain
Do not lock your door with keys if you have defects that you need HDB to rectify. You do not want to come down to open for the personnel.

TOP 10 THINGS TO LOOK OUT FOR ASIDE FROM THE OBVIOUS:

1. Check that all your keys opens your main door properly. Don't forget your letter box key.

2. Turn on all the lights (if it comes with), make sure they're working perfectly. Make sure none of the lights are blown or any of the starters are faulty. If your wiring are hidden as some new flats are, pray that everything is wired properly.

3. Turn on all the taps, faucets, showers and let the water run. If water chokes in your sink or even drain, something's in it's way. Could be plaster etc.

Since BTOs do not come with showers installed, bring a hose along with you, stick it into the sink's tap and flood your floor. If water collects on the bathroom floor and takes forever to drain, your floor isn't slopped properly to encourage proper drainage.

4. Taste the water. It might sound silly, but it should taste... clean, and free from corrosion. Not that stainless steel pipes corrode easily, and this being a new flat and all. But just because the pipes are newly installed doesn't mean the pipes are new (they're not). Bring along a clear drinking glass. Fill it up and see if there are any unnatural particles.

5. Open ALL the windows to their fullest. Check to see if they function properly. Pay attention to the handles to ensure they're not loose or rusty. Also check that they all turn and lock properly. If you have to force the handle into lock position, the handle will loosen up within a short time. This is a problem because most handles are dead bolted onto the frames. And once they are loose, there's nothing to be done but re-bolt them onto another area, leaving unsightly holes on your frame and your handles will not be level with your other windows.

6. Use all the toilets. You don't have to leave behind a souvenir ;) Just flush them and ensure they work properly. Don't just walk away - see how long it takes for the water tank to fill up and flush again. Faulty water tank plumbing may hinder replenishment of the tank's water. Nothing's worse that having a flushing system usable only every 30mins after a night of fish curry, beer and mayonnaise.

7. If your unit comes with a fancy accordion pole hanger for drying clothes, play around with it to ensure everything is bolted properly and its mechanisms are functioning smoothly.

8. If your walls are tiled, take a thick (50c works) coin or marble and start tapping on each of them to check if any of the tiles are hollow or loose.

9. You won't be able to confirm this within 7 days, but be on a constant look out for water leakage\damage on your ceilings after your upstairs neighbor has moved in.

10. Always remember that just because it's a newly built flat doesn't mean everything in it is new. That means piping, gates, windows etc. Check for rust, LISTEN for defects, look for cracks (on the toilet bowls and sinks), and ensure window rubber linings are not dried or rotten.

Source: LaunchPropertySingapore