Normally when building your app, you can use preprocessor directives. When building app for windows phone, VS as default defines WINDOWS_PHONE_APP
(take a look at Project Properties -> Build -> Conditional compilation symbols). Therefore anywhere in your code you can put such a statement:
#if WINDOWS_PHONE_APP
// do when this is compiled as Windows Phone App
#else
// not for windows phoen
#endif
More information you can get at MSDN.
I would advise to use this approach, hence in most cases you know exactly when you will use specific code for Phone (ARM) or other platform. Of course if you need you can define more symbols for specific build configurations/platforms.
Remarks: Since W10, where you need to check the platform in Run-Time, then you can use ApiInformation class and check if specific type exists in the api. For example like this:
if (ApiInformation.IsApiContractPresent("Windows.Phone.PhoneContract", 1))
// do code for mobile
else
// do code for other