Are you doing this manually in the Marketo User Interface or do you want to do this via the API?
To look up in the user interface, use a link like this:
https://app-sjf.marketo.com/leadDatabase/loadLeadDetail?leadId=1
You may have to change the pod (I'm on app-sjf) and of course you need to change the LeadId
If you use the API, you can use
Get Lead by ID (REST API) or
getLead (SOAP API)