I recently purchased a 1998 ford ranger and the heater was not working. The last owner told me the heater core was leaking and bypassed the heater core. I replaced the heater core and the heater only blows cold air. I touched the heater hoses and one of the hoses was cold. I manually open the valve on the heater control vale and it stared to get warm. How do I find know if it is the heater control vale is bad or if its not getting vacuum threw the hose to open the vale? How does one test vacuum on it?

 

Without a vacuum gauge one way would be to remove the hose from the valve with the heat selection calling for max heat and you should feel some suction at the hose end. You could always place a piece of paper over the end to see if it stays there.
Something you might try,
get the higher heater hose where you can just back it off and with the engine running and allow any air that may be trapped in the heater core to get out. Not a good idea to do this when the coolant is hot.