How do you know that there is a bad valve in a Daewoo laganza?

First, the vehicle should be running rough. If you have a bad valve, you will have a low compression situation in the problem cylinder. The only way to check this is to have a compression test performed. After the test the problem cylinder will be indentified. As a tech I would put compressed air into the bad cylinder to see if the air escapes. If you aren't familiar with major automotive workings, this may sound like rocket science. If you had it to a shop and this was what you were told, I would assume it to be true. If you are n't feeling too trustworthy of that shop there is some other things to consider. If you have the rough running condition, you could have a bad ignition component causing the rough run condition. I have seen many mis diagnosis outside of our shop. The only true way is to perform the compression test for starters. This will accurately let you know if you have an internal engine problem.