How do I know if my glow plugs or solenoid are bad. How do you tell the difference.?

Stick a meter on the connection at the top of the glowplug and have someone turn on the ignition. If the meter shows 12V for a few seconds then the feed to the plugs is OK so it's probably the plugs themselves. Take the lead off one and measure resistance to the block. Should be a few ohms. If it's open circuit, there's your trouble.

However, it does sound more like a problem with the relay - it may be that the temperature sender which tells the control unit that the engine needs the heater plugs may be faulty. If the control pack thinks the engine's hot it may not (depends on make/model) apply power.

Run an ammeter across the glowplug busbar, the plugs draw about 8 amps each so 4 cyls should be pulling 32 amps 3cyls 24 etc, substitute a god plug to find the duds.