I m applying for my PhD this fall. But as I started mailing alumni and students of potential thesis advisors to enquire about advisors, I realised it's extremely hassle filled and time consuming, and the worst part is I can ask very few or limited questions about my advisor and that risks my PhD as I wouldn't know.as much as possible about my advisor. What can I do ?

I'm unsure what you mean? Are you emailing other students asking for their experiences with supervisors? Could you not look at publication history, then email potential advisors and see if you can have a chat?

Yes I m emailing prospective students of advisors enquiring about their experience. I have looked at publication history and other stuff and narrowed down a list of 4 professors and am currently talking to the students of two professors and enquiring their experience.

If you are doing this before applying, how do you know a) that you will be accepted and b) that you will get the advisor of your choice? And what if the students tell their advisors about your questions and that puts him/her off you as a student? This just seems like a tactic that could backfire. I think Em's suggestion of a direct contact would be better.

But what if get I get accepted and later when I talk to students I realise, the advisor ain't best fit for me or I ain't best fit for the advisor and the lab ?

It's hard to be 'picky' when it comes to an advisor, as they'll have many students requesting them as supervisors. Your approach, whilst it has merits, may come across as bit egotistical. You do need good advisors, but you won't necessarily know who's the right advisor for another students experience. For example, I know an ex student of my supervisors found the approach really unhelpful where as I found the fact that I was given so much free reign really helped me develop my skills.
You're far better off arranging a chat with potential advisors now, than contacting their students. Have a chat with them, send them your ideas and interests and then see if you're a good fit. You can build the support you want after knowing them, for example you might request they give you deadlines, or you negotiate on how feedback will be given.
Finally, for when you go into the academic world you need to learn how to adapt to work with many different personalities, so whilst it's important to have a good supervisor, I don't think you need to treat it like online dating. Best of luck

Thank you em89, your advice was really helpful. I would follow the same, once again I m really grateful for your advice.