Niacin is a myth started by L Ron Hubbard. Drinking lots of water will not flush you out.

You need to get a home test and test yourself using the first void of the day. Don't go crazy with the water the night before either. If you can pass with a good dark line then you should pass the real thing. If you don't pass then look into diluting or subbing. I like subbing better. If your line is faint you are passing but barely so you might want to do a slight dilute.

Will they detect anything? Yes, they look for dilution by measuring the specific gravity and creatinine clearance of the sample. Too much dilution and the sample will come back negative dilute.

Read the substitution and dilution stickies.