なお、メジャーなフォーマットは以下らしい¹。

• POSIX.1-1988 ustar format
• GNU tar format
• POSIX.1-2001 pax format

利用者から見たフォーマット間の大きな違いは、ustar形式のtarフォーマットの場合、tarでまとめる各ファイルのファイル名が256文字制限だったり、各ファイルのサイズ制限が8GBになってること。残りの２つのフォーマットは制限なし。その代わり、他と比べてustar形式は歴史がある形式なので、対応しているツールも多い。

ちなみに、普通のlinux上のtarプログラム(=GNU tar)がデフォルトで使うtarフォーマットは–helpを付ければわかる。
以下のように、最近のtarプログラムならgnuフォーマットのはず。

# tar --help
[...略]
*This* tar defaults to:
--format=gnu -f- -b20 --quoting-style=escape --rmt-command=/sbin/rmt

・tarファイルのフォーマットの見分け方

hexdumpコマンドでtarファイルを出力し、tarファイル内にアーカイブされた各ファイルのtarヘッダ（とpaxヘッダ)で見分けます。²

hexdump -C で出力された情報のなかで、
それぞれのフォーマットごとに特徴的なヘッダ部分は以下。

• ustar format

  00000100  ...(省略)...  |.ustar.00root...|
  

• GNU tar format

  00000100  ...(省略)...  |.ustar  .root...|
  

• pax format

  00000000  ...(省略)...  |./PaxHeaders.108|
  

以下、各フォーマットの確認例です。

# echo "hello" > foo.txt
# tar --format=gnu -cvf test.gnu.tar foo.txt
# tar --format=ustar -cvf test.ustar.tar foo.txt
# tar --format=pax -cvf test.pax.tar foo.txt

# hexdump -C test.ustar.tar
00000000  66 6f 6f 2e 74 78 74 00  00 00 00 00 00 00 00 00  |foo.txt.........|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000060  00 00 00 00 30 30 30 30  36 34 34 00 30 30 30 30  |....0000644.0000|
00000070  30 30 30 00 30 30 30 30  30 30 30 00 30 30 30 30  |000.0000000.0000|
00000080  30 30 30 30 30 30 36 00  31 31 34 30 35 31 37 34  |0000006.11405174|
00000090  33 30 34 00 30 31 32 33  37 33 00 20 30 00 00 00  |304.012373. 0...|
000000a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000100  00 75 73 74 61 72 00 30  30 72 6f 6f 74 00 00 00  |.ustar.00root...|
00000110  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000120  00 00 00 00 00 00 00 00  00 72 6f 6f 74 00 00 00  |.........root...|
00000130  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000140  00 00 00 00 00 00 00 00  00 30 30 30 30 30 30 30  |.........0000000|
00000150  00 30 30 30 30 30 30 30  00 00 00 00 00 00 00 00  |.0000000........|
00000160  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200  68 65 6c 6c 6f 0a 00 00  00 00 00 00 00 00 00 00  |hello...........|
00000210  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002800

# hexdump -C test.gnu.tar
00000000  66 6f 6f 2e 74 78 74 00  00 00 00 00 00 00 00 00  |foo.txt.........|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000060  00 00 00 00 30 30 30 30  36 34 34 00 30 30 30 30  |....0000644.0000|
00000070  30 30 30 00 30 30 30 30  30 30 30 00 30 30 30 30  |000.0000000.0000|
00000080  30 30 30 30 30 30 36 00  31 31 34 30 35 31 37 34  |0000006.11405174|
00000090  33 30 34 00 30 31 31 30  37 33 00 20 30 00 00 00  |304.011073. 0...|
000000a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000100  00 75 73 74 61 72 20 20  00 72 6f 6f 74 00 00 00  |.ustar  .root...|
00000110  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000120  00 00 00 00 00 00 00 00  00 72 6f 6f 74 00 00 00  |.........root...|
00000130  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200  68 65 6c 6c 6f 0a 00 00  00 00 00 00 00 00 00 00  |hello...........|
00000210  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002800

# hexdump -C test.pax.tar
00000000  2e 2f 50 61 78 48 65 61  64 65 72 73 2e 31 30 38  |./PaxHeaders.108|
00000010  31 2f 66 6f 6f 2e 74 78  74 00 00 00 00 00 00 00  |1/foo.txt.......|
00000020  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000060  00 00 00 00 30 30 30 30  36 34 34 00 30 30 30 30  |....0000644.0000|
00000070  30 30 30 00 30 30 30 30  30 30 30 00 30 30 30 30  |000.0000000.0000|
00000080  30 30 30 30 30 35 30 00  31 31 34 30 35 31 37 34  |0000050.11405174|
00000090  34 30 31 00 30 31 33 34  34 31 00 20 78 00 00 00  |401.013441. x...|
000000a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000100  00 75 73 74 61 72 00 30  30 00 00 00 00 00 00 00  |.ustar.00.......|
00000110  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000140  00 00 00 00 00 00 00 00  00 30 30 30 30 30 30 30  |.........0000000|
00000150  00 30 30 30 30 30 30 30  00 00 00 00 00 00 00 00  |.0000000........|
00000160  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200  32 30 20 61 74 69 6d 65  3d 31 32 37 36 34 34 32  |20 atime=1276442|
00000210  38 37 30 0a 32 30 20 63  74 69 6d 65 3d 31 32 37  |870.20 ctime=127|
00000220  36 34 34 32 38 32 30 0a  00 00 00 00 00 00 00 00  |6442820.........|
00000230  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000400  66 6f 6f 2e 74 78 74 00  00 00 00 00 00 00 00 00  |foo.txt.........|
00000410  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000460  00 00 00 00 30 30 30 30  36 34 34 00 30 30 30 30  |....0000644.0000|
00000470  30 30 30 00 30 30 30 30  30 30 30 00 30 30 30 30  |000.0000000.0000|
00000480  30 30 30 30 30 30 36 00  31 31 34 30 35 31 37 34  |0000006.11405174|
00000490  33 30 34 00 30 31 32 33  37 33 00 20 30 00 00 00  |304.012373. 0...|
000004a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000500  00 75 73 74 61 72 00 30  30 72 6f 6f 74 00 00 00  |.ustar.00root...|
00000510  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000520  00 00 00 00 00 00 00 00  00 72 6f 6f 74 00 00 00  |.........root...|
00000530  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000540  00 00 00 00 00 00 00 00  00 30 30 30 30 30 30 30  |.........0000000|
00000550  00 30 30 30 30 30 30 30  00 00 00 00 00 00 00 00  |.0000000........|
00000560  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000600  68 65 6c 6c 6f 0a 00 00  00 00 00 00 00 00 00 00  |hello...........|
00000610  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002800

1. Pythonのtarfileライブラリのマニュアル参照。
2. ヘッダの概説はFreeBSDのtarマニュアルがわかりやすいです。もっと知りたい場合は各tar形式の仕様書を見た方が良いです。