In most parts of Europe, tips or service charges are included in your bill. How do you know what or how much to tip? You could learn the language and read your bill. You could ask a local before you get to the restaurant. Or you could do a Google search to see what the custom is in the country where you will be eating.
If you do see it included in your bill and your service was outstanding, it is okay to leave an additional tip. But, it’s a good idea to check first. And, Asia is different than either Europe or the US. Many countries there do not even expect any tip.
Bottom line…check first.