someone mentioned one of the hydrocarbon test strips, sounds like a good Idea to me. If it is the gaskets, make sure you get them done by someone familiar with Subaru... If the Bolt torquing sequence is not followed exactly it can cause future issues. and it's about the most confusing one I've ever done..

oh here it is (for the old cars, don't know if it's changed)
i know it's no good without the pictures but just for refrence

Cylinder Head Installation
1. Apply a coat of engine oil to washers and bolt threads.
2. Tighten all bolts in sequence as shown in Figure 2 below to 22 ft. lbs (29 Nm or 3.0 kg-m).
3. Then, tighten all bolts in sequence to 51 ft. lbs (69 Nm or 7.0 kg-m).
4. Back off all bolts by 180° in sequence first; and then back them off by 180° again.
5. Tighten bolts 1 & 2 to 25 ft. lbs (34 Nm or 3.5 kg-m).
6. Tighten bolts (3), (4), (5) and (6) to 11 ft. lbs (15 Nm or 1.5 kg-m).
7. Tighten all bolts an additional 80-90°in sequence.
8. CAUTION: Do not tighten bolts more than 90° at this time.
9. Further tighten all bolts by 80-90°in sequence again.
10. CAUTION: Ensure that the total re-tightening angle [in steps 7 and 9] do not exceed a total of 180° rotation

You get that ?