How can I detect if a web page is opened by a mobile browser or an app?

if there is a web page which will be inserted into an app, it can be linked to another page within an app when I click somewhere on the web page because of the special link that can only be open within an app. However, if I want to make the same web page be opened within a mobile browser, I have to replace the special link.So,the prerequisite is that I need to detect if the web page is opened within an app or a mobile browser. Anyone can help?