On the EOS.io Distribution GUI, they show you how much you have contributed to a particular window in the provided table. If you are not using the official app or maybe it is offline, you can do this through MEW.

1. Go to https://www.myetherwallet.com/#contracts
2. Under heading Select Existing Contract in a dropdown, select EOS Contribution
3. Click Access
4. Under the "Read / Write Contract" heading, in the dropdown select userBuys.
5. In the first field uint256 put the period/window you would like to check.
6. In the field address put the Ethereum address you contributed from.
7. Click Read
8. In the third field, a number will appear if you contributed to that window. Divide that number by 1000000000000000000 with a calculator or google or something.

The resulting number is the amount of Ether you have contributed to that window

Uses

1. Maybe you contributed near the end of a window, and not sure which window your contribution went towards
2. You've previously executed buyWithLimit and just want to check your work.