Two of the pins will have a moderate resistance (10-100 ohms?) between them. Call them heater. If there's a pin with a noticeable capacitance (50 pF or more) against the heater, it's a cathode.
You can probably start with the assumption that the cap is the plate. Not always, but pretty common. Sometimes you can see where the cap wire goes.
The remaining two or three pins are likely to be grids. The screen grid will probably have the highest capacitance against the plate, the suppressor less, and finally control grid the least.
Sometimes there's a second plate, used as a diode. You'll want to check for this if one of the pins doesn't control the main plate current.