[From: firstname.lastname@example.org (Ralph Valentino)]
The most reliable way to tell what type of drive you have is to call the manufacturer with the model number and ask. There is an extensive list of phone numbers in the References section of the FAQ.
That aside, the first thing to check is the number of pins on the drive's connector(s). The second thing to check is the CMOS setup, assuming, of course, that it is in a working system.
SCSI = 1 cable: 50 pins (note 1,2) usually set up as "not installed" in the CMOS IDE = 1 cable: 40 pins no reliable way to tell from the CMOS RLL = 2 cables: 34 pins & 20 pins always has 26 sectors per track MFM = 2 cables: 34 pins & 20 pins always has 17 sectors per track (note 3) ESDI = 2 cables: 34 pins & 20 pins (note 4) usually set up as type #1 in the CMOS and auto-configured at boot time
If you've narrowed it down to RLL/MFM or ESDI but it isn't in a working system, there's no easy way to narrow it down any further just by looking at the drive.
note 1: The QIC-2 tape drive interface also has 50 pins
note 2: To differentiate single ended and differential SCSI, see the scsi-faq
note 3: Some people attempt to set up MFM drives as RLL with varying success, this method will only tell you what it is set up as.
note 4: While ESDI uses the same type of cables as RLL and MFM, the signals are very different - do not connect ESDI to RLL or MFM!