Determining the Qiblah Direction for Salaah

In the enquired case, it was necessary for you to try your level best to determine the Qiblah direction before performing Salaah. It was incorrect for you to follow a group of people assuming that they were performing Salaah in the right Qiblah direction.

If you had tried your level best to determine the Qiblah direction before performing Salaah and you later discovered that the Qiblah direction was incorrect, your Salaah will be valid.

However, if you did not try your level best in determining the Qiblah direction before performing Salaah and later discovered that the Qiblah direction was incorrect, your Salaah will be invalid and will have to be repeated.

In either case, if the Qiblah direction turned out to be correct, your Salaah will be valid and will not have to be repeated. (Shaami 1/433)

And Allah Ta’ala Knows Best