WordPress: Check if plugin is installed (ACF)

ACF itself uses a check to see if the framework has been loaded. If it has already been included and invoked by another plugin or theme, then ACF won't re-instantiate its own class again. It does this with a class check:

if (!class_exists('ACF')) {    //  The ACF class doesn't exist, so you can probably redefine your functions here}

I use exactly this in my own plugins that rely on the presence of ACF so that if it happens to get deactivated, the whole site doesn't bomb out.

First of all, this is not the main plugin function, just one of them. Probably, most commonly used by a plugin user in a theme. Another one is the_field(), which actually prints value (get_field() returns it).

Regarding practice of defining your custom function - it's fine. However, I would not print that long message in every place where ACF field is expected - some of them may be short (numbers), and this message will break the layout. Printing something shorter is better, imo.

Also, function_exists is proper check, not is_plugin_active, because ACF can also be shipped as a library with a theme framework or other plugin.

Another option is to remove ACF dependency on the frontend completely. You can output the contents of the fields with get_post_meta() and prevent ACF plugin from loading on the frontend entirely. See these two posts for details:

http://www.billerickson.net/code/disable-acf-frontend/

http://www.billerickson.net/advanced-custom-fields-frontend-dependency/

Yes, it's a good way to check if the plugin function exists.

You can also try is_plugin_active function to check if the plugin is activated, because the function can be redeclared somewhere.

I think the main reason you are doing that is to prevent Fatal Errors, so it doesn't matter which way you can use.