Check if table or View exists

I am developing an application where I will have a default table and/or
view for a select statement for my application. If a client has a special
requirement I would like to have a alternate table of the same name in a
different schema with the change structure.

Schema A
Default table - coretable.foo

Schema B
Client specific table client.foo

How can I check if a table or alternate view exists in schema B.

ie

if client.foo exists // check for a view or table
// my code will use client.foo table
else
// my code will use coretable.foo
endif

Also is it possible to check via function/via in postgress and not have to
rely on my application to do the check. If so how.