Add formatting check option

tox.ini

@@ -1,6 +1,7 @@
 [tox]
 envlist =
     format
+    format_check
 
 [testenv:format]
 deps =
@@ -11,6 +12,15 @@ commands =
     isort bdfr tests
     black bdfr tests --line-length 120
 
+[testenv:format_check]
+deps =
+    isort
+    black
+skip_install = True
+commands =
+    isort bdfr tests --check
+    black bdfr tests --line-length 120 --check
+
 [isort]
 profile = black
 multi_line_output = 3