To detect if a user has already installed Phantom, a web application should check for the existence of a phantom object. Phantom’s browser extension and mobile in-app browser will both inject a phantom object into the window of any web application the user visits, provided that site is using https://, on localhost, or is 127.0.0.1. Phantom will not inject the provider into iframes or sites that use http://. If a phantom object exists, Solana apps can interact with Phantom via the API found at window.phantom.solana. This solana object is also available at window.solana to support legacy integrations. To detect if Phantom is installed, an application should check for an additional isPhantom flag. 
const isPhantomInstalled = window.phantom?.solana?.isPhantom
If Phantom is not installed, we recommend you redirect your users to https://phantom.com/. Altogether, this may look like the following: 
const getProvider = () => {
  if ('phantom' in window) {
    const provider = window.phantom?.solana;

    if (provider?.isPhantom) {
      return provider;
    }
  }

  window.open('https://phantom.app/', '_blank');
};
For an example of how a React application can detect Phantom, refer to getProvider in our sandbox.