It is very simple to launch the Chrome browser with iPhone user agent string. Changing the user agent string will simulate iPhone browser inside Chrome. You can check how your or particular website will look in Apple iPhone. If you want to change user agent other than iPhone you can do it too replacing the following user-agent string with yours.

iPhone user agent string

Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543a Safari/419.3

Change User agent on Chrome Windows

Step 1:Right click on chrome browser icon and click on Properties.

Step 2: Check target string, add the following string at the end of it.

--user-agent="Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543a Safari/419.3"

Complete string will look like as:
C:\Users\user\AppData\Local\Google\Chrome\Application\chrome.exe --user-agent="Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543a Safari/419.3"

Step 3:Launch Chrome browser clicking the chrome icon.

Done, now browse the site of your choice it as good as you are browsing on iPhone .

Change User agent on Chrome Ubuntu/Linux

Step 1:Go to terminal

Step 2: Type this command.

google-chrome --user-agent="Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543a Safari/419.3"

Done.

You can see different HTML layout and styles if that site has also designed separately for iPhone user agent. Try re-sizing the chrome browser to iPhone size to mimic exact layout and behavior. As chrome is also based on same HTML rendering engine Webkit, the output you see is around 99% similar as you see that site on iPhone.