Doorbell transformer has two black wires and one green. How can i tell which one is hot?

Green is ground. The secondary of the transformer is floating, there is no reference to ground unless you ground one of the two blacks.

If you are replacing an existing transformer where the existing has a white and replacement is two blacks then just pick a wire, mark it with tape, and think of it as white.

"Hot" is a relative term. Voltage between two poles doesn't establish ground and hot. When you ground one wire the other becomes hot.

For instance a battery has two terminals with voltage between, but no reference to earth (or frame of device). Terminal are labelled + and - based on a (incorrect) theory of which direction the electrons flowed.

Early Fords had the positive terminal of a battery connecting to the chassis, establishing the positive as the "ground" (imagined earth) and the negative as the hot. (ASAE were dominated by Chevy fanatics that established negative as ground.)

So in the same way the two black leads are similar to the two ends of a battery. We can't label the wires + or - because of the Alternating feature of AC current that inverses and restores polarity 60 times a second. In many small transformers the is no manufactured connection of either pole to it's frame. Some small transformers have arbitrarily choose to use a white and another color for the leads just because some people expect a color difference.

If you look at RING(tm) wiring diagrams they don't show hot or ground because it is expected that the secondary of the transformer has been left ungrounded.

There are multiple possibilities here.

Black to black will be the maximum output voltage (24v example). Some transformers have a 1 and a 2 or A & B on the wires when connected to #1 terminal to green there is 1 voltage (16v example). #2 Terminal to green is a different voltage (8v example). The 2 different voltages add to the maximum voltage in some cases the voltage is the same 12 & 12 and again the total is 24v So it depends on the type of transformer secondary but black is hot in all 3 cases. How can both be hot? A transformer creates a separate derived source of power in code speak and until a ground point is created we don’t know so we need to know the type of transformer first. A center tapped transformer the center tap is normally grounded. A black to black on the secondary the green is usually not needed and wire nutted either side at that point can be grounded.