How did Alfred find Martha?
In events of Batman v Superman: Dawn of Justice, after the two protagonists were done fighting each other, owing to the fact that their mothers shared the same name, Batman says, he is going after Martha. He did not know where she was, but Alfred did and he lets him know her location.
How does Alfred know where she was? I want to know how did he tracked Martha?
It was shown that they were tracking the Russian man, but how did he know that he kidnapped her?
Because they were already tapping Russian's phone
This is every call made from the Russian's phone. Two mentions of business with the "White Portuguese" and it's continuing transmitting black out data to the personal residence of Alexandra Luthor.
And later after the Martha's reveal
ALFRED: Uh, I'm sorry for listening in. But I've tracked the Russian's phone to a warehouse near the port. You're locked on to it.
So they just have to track a phone location of a person they had doubts.
Refer lyrics.az for full script.