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How can I tell whether my harddrive is laid out using an MBR or GPT format?

You can use parted -l to determine the type of partition table. Eg:

$ sudo parted -l
Model: ATA TOSHIBA THNSNS25 (scsi)
Disk /dev/sda: 256GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number Start End Size Type File system Flags
 1 4194kB 32.2GB 32.2GB primary ext4 boot
 2 32.2GB 256GB 224GB primary ext4


Model: ATA Hitachi HDT72101 (scsi)
Disk /dev/sdb: 1000GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number Start End Size Type File system Flags
 1 1049kB 32.2GB 32.2GB primary ext4 boot
 2 32.2GB 996GB 964GB primary ext4
 3 996GB 1000GB 4295MB primary linux-swap(v1)

The Partition Table field shows that I am using a msdos MBR partition table (the one still commonly used for Linux and Windows) on both disks. From the man page parted can create (and thus hopefully identify) the following types of partition table (or more broadly `disk label'):

bsd
dvh
gpt - this is a GPT partition table
loop - this is raw disk access without a partition table
mac
msdos - this is a standard MBR partition table
pc98
sun

Update

It is worth adding the command for listing a single partition since this is not obvious without some knowledge of parted and it can be a pain finding the data you need if there are multiple drives. For /dev/sda you would do:

parted /dev/sda print

On linux, you can check this via the gdisk tool which should be available for any distro.

gdisk -l /dev/sda

Here, /dev/sda is the device node of the physical drive, not a partition (/dev/sda1, /dev/sda2, etc. are partitions).

If you see something that includes:

***************************************************************
Found invalid GPT and valid MBR; converting MBR to GPT format
in memory. 
***************************************************************

You have a MBR style disk. Don't worry, this did not do any harm.

If you don't see this warning, you have a GPT disk, or a hybrid GPT/MBR disk. The later are used mostly on Apple machines intended to dual-boot versions of MS Windows which do not support GPT. gdisk will indicate this with:

Found valid GPT with hybrid MBR; using GPT

They may also be used in other situations where support for both styles is required.

As the OS was not specified, here is FreeBSD way of doing things.

All is done through the gpart command (short for GEOM partioner - nothing to do with GNU).

A simple gpart show would show you all the available partitions of all the disks, but you can specify the device to have a more precise look on one:

• 
  

  legacy partition layout with MBR (aka "msdos") and BSD partition schemes (a 2-level partitioning was usually required for BSD systems, unless using the full disk):

  
  
  

  $gpart show

  
  
  

  => 63 67108801 ada0 MBR (32G)
   63 67108545 1 freebsd [active] (32G)
   67108608 256 - free - (128k)
  
  => 0 67108545 ada0s1 BSD (32G)
   0 2097152 2 freebsd-swap (1.0G)
   2097152 65011393 1 freebsd-ufs (31G)
  

  
  


• 
  

  modern partition layout using GPT:

  
  
  

  $gpart show /dev/ada2

  
  
  

  => 34 976773101 ada2 GPT (465G)
   34 6 - free - (3.0k)
   40 128 1 freebsd-boot (64k)
   168 67108864 2 freebsd-swap (32G)
   67109032 901775360 3 freebsd-zfs (430G)
  

  
  

To know more, all is in the gpart manual.

With udisks on Linux:

$ sudo /lib/udev/udisks-part-id /dev/sda
using device_file=/dev/sda syspath=/sys/devices/pci0000:00/0000:00:0b.0/ata1/host0/target0:0:0/0:0:0:0/block/sda, offset=0 ao=0 and number=0 for /dev/sda
Entering MS-DOS parser (offset=0, size=500107862016)
MSDOS_MAGIC found
found partition type 0xee => protective MBR for GPT
Exiting MS-DOS parser
Entering EFI GPT parser
GPT magic found
partition_entry_lba=2
num_entries=128
size_of_entry=128
Leaving EFI GPT parser
EFI GPT partition table detected
UDISKS_PARTITION_TABLE=1
UDISKS_PARTITION_TABLE_SCHEME=gpt
UDISKS_PARTITION_TABLE_COUNT=4

Above, I've got a drive with hybrid GPT+MS-DOS partitioning. In that case, the Linux kernel ignores the MS-DOS partitioning, which is why udisks sets UDISKS_PARTITION_TABLE_SCHEME to gpt.

That udisks-part-id tool is used to populate the udev database. So if you've got udisks installed, you should be able to query that information even as a non-priviledged user with:

$ udevadm info -q property -n sda | grep UDISKS_PARTITION_TABLE_SCHEME
UDISKS_PARTITION_TABLE_SCHEME=gpt

Use

$ sudo fdisk -l 

Disk /dev/sda: 119.2 GiB, 128035676160 bytes, 250069680 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x987c1a05


Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 999423 997376 487M 83 Linux
/dev/sda2 1001470 250068991 249067522 118.8G 5 Extended
/dev/sda5 1001472 250068991 249067520 118.8G 8e Linux LVM

See the Disklabel type: dos. If it shows dos that means it is MBR schema else GPT schema

In my Alpine Linux Partition scripts I use:

check_scheme() 
 fdisk -l $1