Conditional to confirm we can rewrite

inc/functions.php

@@ -12,6 +12,31 @@ namespace Camo_Image_Proxy;
  *
  * @return object
  */
-function Options() {
+function Options() { // phpcs:ignore WordPress.NamingConventions.ValidFunctionName.FunctionNameInvalid, Generic.Functions.OpeningFunctionBraceKernighanRitchie.ContentAfterBrace
 	return Options::instance();
 }
+
+/**
+ * Can URLs be rewritten to use Camo?
+ *
+ * @return bool
+ */
+function can_rewrite() : bool {
+	$host = Options()->get( 'host' );
+	$key  = Options()->get( 'key' );
+
+	$can_rewrite = true;
+
+	// Validate host.
+	if ( empty( $host ) || ( ! filter_var( $host, FILTER_VALIDATE_URL ) && ! filter_var( $host, FILTER_VALIDATE_IP ) ) ) {
+		$can_rewrite = false;
+	}
+
+	// Validate key.
+	// TODO: make sure it's an HMAC or something?
+	if ( empty( $key ) || ! is_string( $key ) ) {
+		$can_rewrite = false;
+	}
+
+	return apply_filters( 'camo_image_proxy_can_rewrite', $can_rewrite, $host, $key );
+}