Hi Glen & All,
The differences between the two are revealed by spectra and orbit.
Because all the globular clusters are ancient objects (canonical age limit approx 10GYrs old), they are all very low to ultra low metallicity stars -- with metallicity < 5% that of our Sun. Born into a different epoch of our Milky Way's history when the interstellar medium was not terribly enriched with metals and pretty much the only "stuff" was Hydrogen & Helium. The spectra of the globular clusters is generally similar to the Milky Way's halo stars which for the most part are similarly old.
Also because they are so old, an H-R diagram shows a very low turn-off point on the main sequence -- somewhere around early G. All the higher mass, higher temperature, more short-lived stars have died and gone to God. Then there is a strong red giant branch (not just a few) and most importantly RR Lyrae stars. These RR Lyrae stars (mini Cepheids) are *relatively* blue, periods of < 1 day. These are called "horizontal branch" stars. These low-mass stars have already done one trip up the red-giant branch, have undergone the helium flash, contracted and become bluer before their second visit up the red-giant branch then expulsion of atmosphere and then settling down to become a white dwarf.
https://en.wikipedia.org/wiki/Horizontal_branch
See the two attached images, one is of the G.C M5 which you can see has a short main sequence and low turn-off point, a strong red giant branch (in orange) and a horizontal branch (in yellow). There are no main sequence stars higher than about G4 because all of them have long ago burnt-out. Classically globular
The very different-looking second image is an H-R plot for an open cluster (in this case The Pleiades). Stars on the main sequence go all the way up to about B4 (blue, hot, moderate-mass stars). No red giants (the cluster is too young) and no H-B stars. Classically open.
Lastly, if the cluster's orbit does not coincide with the Milky Way's thin disc (where all present day star-formation takes place), is in the halo and high eccentricity it is almost certainly a G.C because back +10GYs ago star and cluster formation wasn't restricted to the disc of our galaxy and the galaxy looked somewhat different.
Yes, there are some odd-ball objects out there that are usually really old open clusters that look a bit "globular-ish" but not many. Also the very highest mass O.C's and lowest mass G.Cs are about the same size and overall mass. Bigger than a few thousand stars -- very likely globular. Smaller than a few thousand -- very likely open.
So, if the cluster has very low to ultra low metallicity, an H-R plot that shows a very short main-sequence, low turn-off point, strong red giant branch + RR Lyrae stars and it follows an eccentric, high inclination orbit -- that's a Globular Cluster.
If it has a strong and long main sequence, maybe a few red giants, no H-B stars, high metallicity (approaching solar or above) and orbits in the plane of the Milky Way's disc -- that's an open cluster.
Spectra + the H-R plot reveals nearly everything you need to know.
Best,
L.
