How to email current students in graduate programs you're considering?
My email has been posted at both my MSc and PhD department's website. I do not mind cold emails, even before applications are submitted. I answer this question maybe 3-4 times a year before applications are even due: "If you could go back in time and start over, would you still come back?". Applications cost money, I don't mind spending an hour or two every year helping people whose shoes I was once in. 

Most people are the same I'd think.

Edit: 

I'll read anything with my advisor's name in the email, so, if you title the email "Prospective student for Advisor" or something like that, then It will probably at least get read.