For this type of question I get my pupils to write a chart on the rough paper first. First they decide on the key and then work out what the four possible chords are, writing them as four rows. Once they have done this they put the letters a, b and c at the top of the 2nd, 3rd and 4th columns. Then it's just a case of looking at the notes in the music to decide which chord you are looking at. Once that is done you look for the lowest note in the chord in the music. You find that letter in your table and then look up to the top of the column, where you will find whether it is root position (a), 1st inversion (b) or 2nd inversion ©.
Here's an example table:
C major
a b c
I C E G
II D F A
IV F A C
V G B D
If the chord was (from bottom upwards) A, C, F, C. You would see that it is the subdominant chord. Finding the A in the row, you would look to the top of that column and discover that you need to write b after the number. Thus the chord is IVb.
Full marks here we come!
EDIT: The above chart isn't very good. As the inversion letters are in the wrong place. When it's written out or using Publisher it works a treat!