If someone tries to register a similar trademark, how will I know about it?

If you own a trademark, a new application for a mark confusingly similar to yours can cause consumer confusion in the market and, therefore, does constitute infringement. Thus, you can use your ownership rights to oppose the mark during the opposition window.

So, how can you learn about these applications? IP offices do a part of the work for you, for example:

• Some jurisdictions, including the EU and the UK, will notify you if the examiner evaluates a new mark and considers it too similar to yours. Your trademark is in their register, and comparing the new mark against all other registered trademarks is a part of the registration process.
• In the US, this goes even a bit further. The USPTO won't notify you but will outright stop the registration of the other mark during the examination process.

However, both of these cases have limitations. They both depend heavily on the examiner's reading of the two marks. There might be marks the examiner hasn't considered, but you might deem them your competition and want to bring them to the examiner's attention. In the case of the US, the other applicant might successfully convince the examiner their trademark should be registered, even though you might want to add evidence to the contrary. However, that debate would happen solely between the examiner and the other mark's owner; you won't be invited to participate.

Therefore, considering a trademark watch service might be a good idea. These services monitor trademark registers for similar trademark applications based on certain criteria and then notify you about them via email.