How do I know if I have 240v when replacing old baseboard heaters

Run the wire to the thermostat then to the heater

BASEBOARD WIRING WITH A SINGLE POLE (2 WIRE) WALL THERMOSTAT - 120V OR 240V SUPPLY

BASEBOARD WIRING WITH A DOUBLE POLE (4 WIRE) WALL THERMOSTAT - 120V OR 240V SUPPLY

Here is a link to the wiring instructions, which has detailed pictures and instructions. This should make it a lot easier. http://www.marleymep.com/en/multimedia-library/pdf/fahrenheat-pdf/products/f2500-series/F2500wiring.pdf

You need 240 breaker:
Notice that your 240V breaker is twice as wide as regular 120V breaker.
Notice that both switches are tied together on 240V breaker, making this a double-pole breaker.
For more information about breakers and 120-240V circuits:
http://waterheatertimer.org/See-inside-main-breaker-box.html
http://waterheatertimer.org/B220C.html

Using the equation I = P/E where "I" is Amps, "P" is Watts, and "E" is voltage, we find that your 5000w circuit will have an Amperage of 20.83. A 22 amp tstat will work, it will just leave you no room for surges. I would suggest splitting the baseboard and using 2 thermostats. Hope this helps

Single Pole is 120v
Double Pole is 240 volts

on the one that don't work use a clamp around ammeter and see if you are drawing current. no current, the heater element is shot!

you get 120V from any phase (black OR red) to neutral
you get 240V from phase to phase (between black and red) and in this scenario you don't need the white wire.
So for you, connect the black to 1 pole of the breaker and the red to the other pole, connect the green wire to the ground strip and you are done.
A white wire is NEVER connected to a circuit breaker anyway, it is a grounded wire, it would be like connecting a green wire to a black wire, bad idea.
the National electrical code specifies red and black wire color for 240VAC circuits.
As Ask Hank mentionned, nothing wrong with 10/3 wire.

no does not matter but you must have a ground leg

I can help you if you still here.

It sounds like you are not getting the correct voltage for the heating element. If you wired a single pole thermostat, and its a 220v unit, then it will not get hot enough, like you are saying. If its a 220 heater, you need a 2p2t thermostat. The parts supplier will know what this means.