re: "how do i look for certain clients?"
I will restate your question in my own words:
Question: "I know the name of a client. For example (using a made-up name): Peter McDanvers-Gentry. How do I SEARCH for this client?"
Answer: If you have already been hired by a client in the past, then you can open your Upwork Messages tool and use the search field in the top left hand corner of the screen:
This will let you find conversations in the Messages tool that you previously had with that client.
You can go here:
Upwork -> My Jobs -> All Contracts
You will see a SEARCH field you can use to search for the names of clients who you have worked for in the past: