How to find what SQL Jobs are using a specific account as Proxy?
The servers I was looking at had only few credentials. While they had several jobs only a handful were using “Run As” Proxy credentials.  This is the solution I used.

First query shows the account (credential identity) linked to the proxy
Second Query shows what Job and Step is using a Proxy

    -- Search Credentials (shows account for Name)

    use msdb
    select *
    from sys.credentials

    --Search Jobs where there is a 'Run As' proxy and get the name of that proxy

    use msdb

    select  sysjobsteps.job_id
    , sysjobs.name as 'JobName'
    , sysjobsteps.step_id
    , sysjobsteps.step_name
    , sysjobsteps.subsystem
    , sysjobsteps.last_run_date
    , sysjobsteps.proxy_id
    --, sysjobsteps.step_uid
    , sysproxies.name as 'ProxyName'


    from sysjobsteps
    left join dbo.sysproxies
     on sysjobsteps.proxy_id = sysproxies.proxy_id
    left join dbo.sysjobs
     on sysjobsteps.job_id = sysjobs.job_id

    where sysjobsteps.proxy_id &gt; 0


I tried a couple of ways of joining sys.credentials to dbo.sysproxies. If there was more than one credential it did not work well. Separate quires met my needs so I did not dwell on the join.