Dave.
If you want to check the total current that the valve is drawing, then just disconnect the cathode bias resistor at the valve base and connect your meter between the disconnected end of the resistor and the valve base contact (positive to valve base, negative to resistor). Set your meter to a suitable range (say 50mA) and it will indicate total valve current (made up of anode current and screen grid current).
If you just want to measure the anode current, then disconnect the HT feed to the primary of the output transformer, connect your meter (negative to transformer, positive to HT feed), set your meter to a suitable current range (say 50mA) and it will show the anode current only. BE VERY CAREFULL when doing this, first because of the high voltage and second, if the meter does not make proper contact, there is a danger that the valve will run without anode voltage and the screen grid will get red hot.
Don't worry! Just take your time and check your connections first.
Rich.