Without more facts, we can only tell you to see an experienced tax professional.
LLCs are state-level business designations. Each state had different rules on how to form an LLC and what paperwork is required. The IRS does not recognize LLCs as a separate kind of tax entity. If you are a single-member LLC in your state, you file a federal tax return as a sole proprietor on schedule C. If your LLC has more than one member, you file your federal tax return as a partnership.
S-corp is a federally designated type of business with uniform rules across all states. There are specific procedures you must use to create an S-corporation.
There is also a hybrid option--you can have an LLC but choose (elect) to file your tax returns as if you are an S-corp. This requires filing a specific form with the IRS.
So ultimately, your question makes no sense. You can't be an LLC or an S-corp without filing specific paperwork when you create the business. Only you know what you actually did when you started the business.
