Nur, when you see a job in Find Work (My Job Feed), click on the job, then look under Activity on this Job. If it lists any 'hires', then someone has already been hired. You can still apply and the client may hire you, but unless the job specifies that it's looking for xx freelancers, you probably will not be hired.
Here's an example of a job that already has 2 hires:
"No good deed goes unpunished." -- Clare Boothe Luce