I put the Neon install iso onto a usb stick.

Is there any way to tell whether it was created as UEFI or legacy BIOS? My motherboard keeps loading it as UEFI, and seems to only see it as UEFI.

This is strange as the drive was formatted and created in a CentOS terminal.

I'd like to install Neon on the same SSD as a CentOS install, but early in the install menu I get a conflict warning that the install medium was loaded as UEFI, and if I install Neon, I won't be able to dual boot.

This happens even if I load the live install USB as OEM. Is the USB drive UEFI or not? How can I tell?

it can boot both legacy bios and uefi.
At power on, bring up your system's bios boot menu and see if it shows up as a uefi boot option as well as a legacy one.

Took hours, but finally may have found it under 'hard drive BBS priorities,' where it seems to only offer a single legacy boot option (I had to move the usb stick to priority over the BM CentOS boot option).

*Not a fan of GIGABYTE GA-X99P-SLI's BIOS menu.