The best way is to use another special move on Raikou that has a power anywhere around 30-70 then judge how much damage HP does compared to that.
Save before you add the moves so you can keep the old moves if it doesn't work out.
For example, teach it Signal Beam, power 75. Go out in battle and see how much damage SB does, then see how much damage HP does (on the same pokemon), and compare. If HP does almost as much damage, HP's power is close to 70, if it does only about half the damage then its power would be closer to 30.
Repeat a few times to make sure you got it right. I did this on my Ampharos since I already had signal beam and hidden power was really weak. Also make sure both moves you test are not super-effective or not very effective.
Shxatoap's answer can work too (use serebii's IV calculator) but you need to know EXACTLY what IVs you have. A difference in 1 IV can change the power from 30 to 70.
