To check if your system is using UEFI boot, you can use following method.

Method 1: efibootmgr

On CentOS 7, efibootmgr is provided by package efibootmgr.

This server is using UEFI

[root@server ~]# efibootmgr
BootCurrent: 0005
Timeout: 1 seconds
BootOrder: 0000,0004,0005,0006,0007,0008,0009,0002
Boot0000* grub
Boot0002  UEFI: Built-in EFI Shell
Boot0004* UEFI: Intel(R) I350 Gigabit Network Connection
Boot0005* UEFI: IP4 Intel(R) I350 Gigabit Network Connection
Boot0006* UEFI: IP6 Intel(R) I350 Gigabit Network Connection
Boot0007* UEFI: Intel(R) I350 Gigabit Network Connection
Boot0008* UEFI: IP4 Intel(R) I350 Gigabit Network Connection
Boot0009* UEFI: IP6 Intel(R) I350 Gigabit Network Connection
[root@server ~]# 

Here is a server that is not booted with UEFI

[root@server1 ~]# efibootmgr
EFI variables are not supported on this system.
[root@server1 ~]# 

Method 2: dmesg

Here is a server booted using UEFI

[root@server ~]# dmesg | grep EFI
[    0.000000] efi: EFI v2.40 by American Megatrends
[    0.000000] ACPI: UEFI 00000000798a3080 000042 (v01 SUPERM SMCI--MB 01072009      00000000)
[    0.936744] fb0: EFI VGA frame buffer device
[   12.109532] EFI Variables Facility v0.08 2004-May-17
[root@server ~]# 

Server not using UEFI boot

[root@server1 ~]# dmesg | grep EFI
[    0.000000] ACPI: UEFI 000000006dd52930 00042 (v01                 00000000      00000000)
[root@server1 ~]#

Method 3: Checking grub.cfg

Here is a server that use UEFI

[root@server ~]# cat /boot/grub2/grub.cfg | grep -i efi
if [ -s $prefix/grubenv ]; then
    insmod efi_gop
    insmod efi_uga
  set locale_dir=$prefix/locale
if [ -f ${prefix}/user.cfg ]; then
  source ${prefix}/user.cfg
	linuxefi /boot/vmlinuz-3.10.0-693.2.2.el7.x86_64 root=UUID=908d7686-177e-42b8-856b-501445522a61 ro  
	initrdefi /boot/initramfs-3.10.0-693.2.2.el7.x86_64.img
		linuxefi /boot/vmlinuz-3.10.0-693.2.2.el7.x86_64 root=UUID=908d7686-177e-42b8-856b-501445522a61 ro  
		initrdefi /boot/initramfs-3.10.0-693.2.2.el7.x86_64.img
		linuxefi /boot/vmlinuz-3.10.0-693.2.2.el7.x86_64 root=UUID=908d7686-177e-42b8-856b-501445522a61 ro single 
		initrdefi /boot/initramfs-3.10.0-693.2.2.el7.x86_64.img
		linuxefi /boot/vmlinuz-3.10.0-693.2.2.el7.x86_64.debug root=UUID=908d7686-177e-42b8-856b-501445522a61 ro  
		initrdefi /boot/initramfs-3.10.0-693.2.2.el7.x86_64.debug.img
		linuxefi /boot/vmlinuz-3.10.0-693.2.2.el7.x86_64.debug root=UUID=908d7686-177e-42b8-856b-501445522a61 ro single 
		initrdefi /boot/initramfs-3.10.0-693.2.2.el7.x86_64.debug.img
		linuxefi /boot/vmlinuz-0-rescue-1e98a4cc1263468b9f9a12e365316b95 root=UUID=908d7686-177e-42b8-856b-501445522a61 ro  
		initrdefi /boot/initramfs-0-rescue-1e98a4cc1263468b9f9a12e365316b95.img
		linuxefi /boot/vmlinuz-0-rescue-1e98a4cc1263468b9f9a12e365316b95 root=UUID=908d7686-177e-42b8-856b-501445522a61 ro single 
		initrdefi /boot/initramfs-0-rescue-1e98a4cc1263468b9f9a12e365316b95.img
elif [ -z "${config_directory}" -a -f  $prefix/custom.cfg ]; then
  source $prefix/custom.cfg;
[root@server ~]# 

Boot entry will be linuxefi, initrdefi instead of linux16/initrd16

Here is server that is booted with BIOS

[root@server1 ~]# cat /boot/grub2/grub.cfg | grep -i efi
if [ -s $prefix/grubenv ]; then
    insmod efi_gop
    insmod efi_uga
if [ -f ${prefix}/user.cfg ]; then
  source ${prefix}/user.cfg
elif [ -z "${config_directory}" -a -f  $prefix/custom.cfg ]; then
  source $prefix/custom.cfg;
[root@server1 ~]# 

Method 4: /boot/efi

Check for mount /boot/efi

[root@server ~]# df -h --local | grep /boot
/dev/nvme0n1p1  510M  3.7M  507M   1% /boot/efi
[root@server ~]# 

If your server use BIOS, you won’t see /boot/efi