If the airline employees do their job right they won't let you board/check-in at your departure location.

I'm not a legal or visa expert so this is just my own opinion without any guarantees that I'm right.

In case they don't do their job right and you get on board and border control agents check your passport for example before you enter the transit area and say you need a transit visa the airline will be fined and you will be probably sent back to country you came from.

Otherwise please read this very clear instructions on the website of the German embassy in Washington, D.C. It is very cleary written and explained who needs a transit visa in general and which additional rules apply for exceptions:
German Transit Visa Rules

For Indian citizens the rule says:

You need a transit visa

except:

1. If you hold a valid visa for the United States of America
2. If you return from the USA after having used the visa (however,
the return has to occur within latest four days after expiration
of the validity of the visa)
3. If you hold one of the following residence permits:

* Form I-551 permanent resident card (valid for 2 to 10 years),
* Form I-551 Alien registration receipt card (valid for 2 to 10 years),
* Form I-551 Alien registration receipt card (no expiry date),
* Form I-327 Re-entry document (valid for two years — issued to
holders of a I-551),
* Resident alien card (valid for 2 or 10 years or no expiry date.
This document guarantees the holder’s return only if his stay
outside the USA has not exceeded one year),
* Permit to re-enter (valid for two years. This document guarantees
the holder’s return only if his stay outside the USA has not
exceeded two years),
* Valid temporary residence stamp in a valid passport (valid for one
year from the date of issue).

So to make it short. Do you have a visa or resident card for the USA?