how can i know my friend is online if they were invisible

Hello Guys...
Some people set their Status Invisible.... for example... I use Yahoo Messanger, I have so many friend in the list.... And some of them may set their Status to be invisible..... how can i know.... he is on the net.... he only set his Status Invisible....
Note: OPening their profile may not solve my problem..... I want another hint.... Plizzz give my a suggestion.... then i can continue on my own....

I hope u get my point.... am very bad in english....


God Luck.. everyone.... and i hope.... ya' reply message....
Here is my mail add... any one whos....intresting my question plizz mail me..... at [email protected] May be i have somethin' 2 tell u....