How do you deduce the oxidation state of these few elements in a chemical equation?

You work on the assumption that H is always +1 in a compound and oxygen is -2 in a compound.  Metals will have an oxidation state equal to to the "ionic" charge it would have if it were in solution as an ion.  The oxidation number of polyatomic ions are well known.  (i.e. sulfate is -2)  Lastly, the oxidation number of any element in the elemental state is zero.

The sum of the oxidation numbers of all of the elements in a comopund is zero.  The sum of the oxidation numbers of all the elements in a polyatomic ion is equal to the charge on the ion.

With all of that in mind we can easily assign oxidation numbers.

 +2 +6 -2 ...........0...........+1+6-2 ............. +3..+6-2 ....... +1-1
2FeSO4 (aq) + Cl2 (g) + H2SO4 (aq) ---> Fe2(SO4)3 + 2HCl

2FeSO4 (aq) + Cl2 (g) + H2SO4 (aq) ---> Fe2(SO4)3 + 2HCl
Oxidation states
 Fe +2  SO4 -2   Cl 0      H+1 SO4 -2        Fe 3+  SO4 -2    H +1 Cl -1

Cl normally -1 in compounds , element 0 by definition
Fe  is +2  or +3  ... compare with sulphates  [-2 overall]