I met this professor last year from online search and expressed my interessts to his research and his advanced lab technique, which i lacks of experience.

I also asked for a phd opportunity from him, he says that he has no money or limited funding now, so i agreed to work for 3 months for free. Would be practical that this volunteer experience could lead to a phd after this 3 months? On one side i am not happy with work for free, as i had employment for 1 year as a research assistant. on the other side, this potential professor appears nice to employees around and keep his words...and have good tracks of papers...

but am i just wasting of time,what actions should i take to figure this out before too late?

Personally I don't think its a good idea. Even if you spend three months working for free there's no guarantee you'll be offered a funded PhD position, so you could just end up depleting your saving for no benefit. You say you already have one year of paid research assistant work experience, so perhaps you can find another RA position for now; and ask the supervisor you want to do a PhD with to contact you if funding becomes available. This way you'll be getting paid work experience instead of voluntary. Maybe your chance of getting the PhD when funding comes up might be slightly reduced compared with if you had been working with the supervisor already, but to me taking the 3 months unpaid work would be too much of a gamble.

Thank you for your reply, Malik. What is the motivations for the professor to ask to work for free, he agreed to apply for funding but it needs a longer period. he has many publications.... on the other hand i can't afford to burn out the money to work for free for a longer period...difficult to find a phd at another place due to limited skills and networks