add "fmt-check" make target

Makefile

@@ -3,6 +3,7 @@
 #   all:          Builds the code locally after testing
 #
 #   fmt:          Formats the source files
+#   fmt-check:    Check if the source files are formated
 #   build:        Builds the code locally
 #   vet:          Vets the code
 #   lint:         Runs lint over the code (you do not need to fix everything)
@@ -11,6 +12,9 @@
 #
 #   install:      Builds, tests and installs the code locally
 
+GO_PACKAGES ?= $(shell go list ./... | grep -v /vendor/)
+GOFILES_NOVENDOR = $(shell find . -type f -name '*.go' -not -path "./vendor/*" -not -path "./.git/*")
+
 .PHONY: all fmt build vet lint test cover install
 
 # The first target is always the default action if `make` is called without
@@ -19,7 +23,10 @@
 all: fmt vet test install
 
 fmt:
-	@gofmt -s -w ./$*
+	@gofmt -s -w ${GOFILES_NOVENDOR}
+
+fmt-check:
+	@([ -z "$(shell gofmt -d $(GOFILES_NOVENDOR) | head)" ]) || (echo "Source is unformatted"; exit 1)
 
 build:
 	@go build