add Check

Check is a function that reports whether a peekable reader looks like it
starts with an ID3 tag.

decoder.go

@@ -7,6 +7,19 @@ import (
 	"io/ioutil"
 )
 
+type Peeker interface {
+	Peek(n int) ([]byte, error)
+}
+
+// Check reports whether r looks like it starts with an ID3 tag.
+func Check(r Peeker) (bool, error) {
+	b, err := r.Peek(3)
+	if err != nil {
+		return false, err
+	}
+	return bytes.Equal(b, Magic[:]), nil
+}
+
 type Decoder struct {
 	r io.Reader
 	h TagHeader

id3_test.go

@@ -1,7 +1,9 @@
 package id3
 
 import (
+	"bufio"
 	"bytes"
+	"strings"
 	"testing"
 	"time"
 )
@@ -128,6 +130,27 @@ func TestUserFrameNameParsing(t *testing.T) {
 	}
 }
 
+func TestCheck(t *testing.T) {
+	tests := []struct {
+		in string
+		ok bool
+	}{
+		{"ID3stuff", true},
+		{"NotID3stuff", false},
+	}
+
+	for _, test := range tests {
+		r := bufio.NewReader(strings.NewReader(test.in))
+		ok, err := Check(r)
+		if err != nil {
+			t.Errorf("Unexpected error %q", err)
+		}
+		if ok != test.ok {
+			t.Errorf("expected ok = %T for %q, got %T", test.ok, test.in, ok)
+		}
+	}
+}
+
 func BenchmarkISO88591ToUTF8(b *testing.B) {
 	b.SetBytes(int64(len(ISOTestString)))
 	for i := 0; i < b.N; i++ {