All wrapped up in plastic, how to tell??

The LED test / Back light  test looks legit .
How bout all the buttons function  is it working as it suppose to ?

Where did you got it from?
There are many fake DW-6900 series out there now..
The batch number might not be on all the back case iinm..

2 most important to recognise fakes 

1) LED Backlight test 
2) All buttons functioning 

The fakes ones  won't be able to imitate or copy those to the letter. for example if the watch have Barometer Sensors when you press the watch button it won't bring up the Barometer ... its just do nothing  .

Back light  LED test will be poor quality and you can sense that its just a cheap light. 

Correct the batch number will not be in all watches especially if the watches are not from Japan factory . So far Japan version mostly have complete batch numbers .

The ones from China or  Thailand factory don' necessary have it in place.

A.I.O is  reliable but you have to check pricing as well. I got the Gravity Master from them and its legit and the real thing 100% Original. Having said that if you troll their shop there are a few that does not quite jive with the market pricing point.
Always do a sanity check on the pricing though for example if you see a Frogman model that sells for 800 or  below  300 RM you instantly know that those are fakes.  Entry level of Frogman price are  3K and above. 

Shoppee have a lot of them selling G Shock Frogman for RM350 normally they put up as sale or sale clearance. When people read the sale discounts they immediately believe that its original so the rest you can put two and two together right. If its too good to be through its a definite Fakes.

See th ghosting number on the digital display ...

Based on my experience, the fake ones don't give you the proper tag with the correct model number..

And the box normally won't have the model stickers too..

No need to return.
As long as there is no obvious fake indication then it should still be legit.

Some watches do not have SN engraved on the back cause that specific part is replaced/refurbished or produced as replacement after production ended.

Just google and can find the answers yourself