Okay, say I bought some sulfuric acid drain cleaner, it contains pure sulfuric acid except some stuff to stop it corroding the drains. It won't specify what chemical.

1. Any idea on what the chemical is?

2. Is it likely to be toxic/carcinogenic?

Usually its some shitty dies or other crap. Toxic carcinogenic? Probably not, but if your trying to clean your H2SO4 i would recommend to boil it down until you get some white fumes appearing :) It works like a charm for me :) Or you can just buy some clean H2SO4 from a chem supplier too, i believe that most hardware carry H2SO4 "clean", just look a little harder :)

Usually you can contact the company and get an MSDS. At least that's been my experience.

Activated carbon / molecular sieves / distil / buy reagent grade.

Arguably in order of effectiveness as well there, purely by chance though.

You can get the MSDS though, which is useful, I mean if you wish to use it in simple synths, nothing that needs to be clean and pure, its probably as good as reagent grade and a quarter of the price. Don't use nasty dyed up pre thickened sulphuric for making meth or whatever though.

If you can't get pure H2SO4 your best bet is to:

1) Boil it down to until white fumes appear
2) multiple washes thought activated carbon
3) or get some molecular sieves ( great to concentrate it)

I mean if you wish to use it in simple synths, nothing that needs to be clean and pure, its probably as good as reagent grade and a quarter of the price. Don't use nasty dyed up pre thickened sulphuric for making meth or whatever though.
Good enough.

While this is up, how does one get the MSDS for a particular product? Call the 1800 on the package? email the producers? Probably a dumb question, but it's always better to ask...

Put the name into google and add msds at the end:
e.g.

"Rooto msds" or "Rooto MSDS"

Or usually if you go on the website of the company it should be there.

I only resort to call them up and ask them to email me the MSDS if i really can't find it, but usually a bit of searching and switching key words should yield what you want ;)

Do a little reading on google hacking, it has proven for me to be very useful to find this sort of thing :)