How can I check if a domain name has been registered as a trademark or not?

As many of the answers have pointed out here, there are several online searches you can use. However, I would urge you to utilize the skills of a professional trademark attorney.

Register trademark in Vietnam

I suggest this because there are many nuances to conducting a thorough trademark search. Conducting a search on your own is an option but you’d be surprised at how many people make mistakes throughout this process. In order to ensure that it’s a thorough search, you are better off relying on people who can ensure that the domain name has not already been registered. Keep in mind, not all protected trademarks may not appear in th TESS (Trademark Electronic Search System).

Ultimately, this could save you valuable time and money.

Hope this helps!