Is there a way to see the custom fields in Marketo you already made so you don't repeat one when making new ones?

May not be ideal, but you can export a full list of fields that are sorted alphabetically, so you could search there to make sure you're not creating a dupe field.
Just go to Admin > Field Management > Export Field Names

It won't tell you which ones are custom, but this ought to be close enough.

but you can do what Drew says to visit Field management to check on field names and types.

Drew and Josh mentioned the best approach here. One resource that might be helpful is the page below to compare which fields are custom to which ones are standard fields in Marketo:
http://developers.marketo.com/documentation/faq/list-of-standard-fields/