I dunno about "fruity"- GBL tastes like you'd expect industrial-strength floor cleaner to taste. (And not the lemon fresh kind.)
One way to test it prior to dosing is to stick the bottle in the freezer for an hour. If any part of it freezes, it's either been diluted or it's another (probably water-based) product altogether.
You can also shake it up- GBL is kind of like a transparent milkshake (high viscosity, or "clingy in the mouth"), and no bubbles should form.
If/when you decide to take it, wait AT LEAST an hour to gauge the effects before taking more. (Any experience with GHB? Effects are comparable, but GBL is about 50% stronger, and it'll take you twice as long to start rolling.)
That delay, obviously, can make it super dangerous. A good friend of mine decided to take more when he couldn't feel his first dose after 45 minutes. He went outside to check his mail, then blacked out and keeled over in his driveway. His neighbor tried and failed to revive him, and he woke up bloodied and bruised in the hospital the next morning.
Which brings me to my final point: DON'T do this by yourself, especially your first time. Seriously.
And, um, sub-finally-- don't drink alcohol for at least 12 hrs prior. That might sound over-cautious, but no joke- Google that shit and take heed.
Good luck!