I know HPS lamps are easily recognizable because of the color. But how can you tell MV from MH lamps? Sometimes the MV lamps are greenish, but not always.

Thank you

Try to take a picture with your camera with the white balance set do daylight (or a neutral colour object illuminated by them if they're too bright).
MV(MBF) lamps will look greenish in the picture even if they're new and look white for your naked eye.

If you can see thee arc tube (while off) the arc tube on the mercury vapor is longer and metal halide is shorter.

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Looking at unlit Mercury lamps vs MH ones-the Mercury lamp arc tubes usually don't have the white heat reflective coatings on the ends of the arc tubes.Sometimes a very slight mirror like coating on the ends of the mercury tube.MH bulbs have the white reflective coatings,and you see the halide salts inside the arc tube-white or slight yellow in color.Have some 400W 6K aquarium bulbs that have light blue color halide salts in their arc tubes.These bulbs take like almost 10 min to warm up to brightness and color.The mercury light warms up in less then 3 min.
 When lt-the mercury is the bluish white color when the lamp is new.As it ages-you see the greenish color.MH are more white-but then again their color depends upon the mixture of halide salts in their arc tubes.Venture Lighting makes MH lamps that have saturated green,blue,pink,or magenta colors.Used for decorative lighting.These bulbs are VERY expensive.Just seen pictures of them lit.They do have nice colors.-But als shorter life than other MH lights.They came in 100W(Pulse start),250W,400W and 1000W probe start.Beleive they are listed in double end 150W.Would like some in my collection-but their high cost-and they have to be custom ordered.

One way to distinguish when looking inside onto the arctube and the arrangement arround:
No auxiliary probe => pulse start lamp => MH (with the only exception of the Eye Moon Pulse)

Starting probe has some thermal contacts => MH (the voltage between the auxiliary and main probe would cause destructive electrolysis when the liquid MH salts would reach that area, so the probe is either disconnected, or more frequently shorted with the main electrode after the lamp ignites and warms up a bit by the thermal contact; and as that is usually one of the coldest spot, they tend to condense just there)
Just a resistor connected to the starting probe => MV lamp (nothing the only conductive liquid could be the mercury, but that means hard short, so no electrolysis)

Metal work around the arctube covered by insulating sleeves, isolated from the circuit or not used at all (using a thin fly wire going clearly as far as possible from the arctube to carry current to the crown end) => MH (the photo induced emission from these structures cause electric field, which tend to attract sodium ions from the arc core into the arctube quartz wall, causing it's blackening and failure, so those measures are intended to prevent that).
MV's have no sodium, so no need to complicate their design by those measures.

Then not that much reliable indications:
MV'a tend to have longer arc, so larger arc tube for given wattage than MH. But some modern MV's use higher arc loading (so smaller arc tube) design as well, the same older MH's used lower arc loading (larger tube) too.
White powder coating is nearly all the time a phosphor on the MV, so glows pink (only the phosphor, not the tube). But many MH designs use the same phosphor as well (some older just because it was readily available white diffusing material, ready with all the application machinery at the factory floors). So only when there is no pink glow, it is most likely MH. But it can not be said the opposite.

Many MH lamps turn to a pinkish color cast when they age.Otherwards to a "warmer" color-and dimmer.Some MH lamps opposite-they start with a warm color and as the light ages-goes to a high color temp and dims.I have some 175W GE "Halarc" lamps that it states on the package they go to a higher color temp as they age.The GE halarc bulb is a pulse state lamp that can run from a probe start ballast.The ignitor is built into the bulbs base.Don't know if they are still made.They make a nice,crisp,warm light-better than a usual 175W MH light.At one time one of the shopping centers was lit by these when GE came out with them in like the '80s.The shopping place was near Wash DC.