I'm on my 3rd vial and the 20 injections from the first 2 gave me a very consistent "pain" (pain is probably too strong a word - "feeling of discomfort" is perhaps more correct).

I would be able to feel where I injected immediately and for about two days after. It would just be slightly... troublesome.

Now, on my 3rd vial I've injected twice and have had absolutely no pain at all - it's literally like I've injected nothing.

So, of course I suspect I've been sold a fake vial.

Is there some simple "test" (pardon the pun) I can perform to check its legitimacy (or otherwise)?

Some more info - vial 3 looks different from the first 2 vials (it has a stronger colour), but then again, the first 2 (which I believe to be legit based on results) looked different from each other too.