C#
The answer is: it depends.
It depends on the programming language you are using and partially on the database you are using.
Most of the databases are services running on a web service. We can communicate with them using a special protocol, while we know the address and the port of the server where it runs. There are also databases stored in files, where we specify the target file.
Most of the programming lanugages and frameworks provide a "Connector" for databases, which allows us to communicate with the database. Of course you need to check if your programming language has a driver for the database type you want to connect to. Some frameworks or programming languaes also offer a higher level control structures called object-relational mapping, where the user can easily read or write some objects defined in the project.
There are many different possibilities to answer this question. We will go top down beginning with the browser.
Most of the browsers support the html input tag of type "email" and most of them would automatically warn the user about a wrong format of the address. However there are many standards of email address pattern. Furthermore the user might disable the browser validation, so we should not rely on this.
The second station is the javascript of the browser. Probably the easiest way to validate the email address is to use a regular expression an match the input against this expression. However the user may disable the javascript or send a request in other way than browser. Therefore it's absolutely necessary to validate the email address on the server side. This might be done by a regular expression, but some more sophisticated systems would check the MX record of the domain given in the email address, to be sure it might be a real address of a real mail server.
Use automated UI Test, which is possible with the Selenium library and a browser driver. The browser driver allows us to run actions in a browser while writing code. The Selenium library is an interface to connect many browsers with the same code base. So now we are able to code a simple test, where a robot is automatically clicking through the website and trying out the login functionality. This technique may also apply to test other important and high priority UI features.
