Most of the time the number listed on the p99 wiki is Less than the actuall HP number.
As swish said, turn on your log file /log on and add up all the dmg you do to it before it dies. That's what I did for A Giant Skeleton. The wiki listed it having 704 hp, I found they have 900-1300 hp.
http://wiki.project1999.com/A_giant_skeleton
There are also programs such as EQCompanion & Yalp11 which can show you fights/total dmg done. but look luck finding them I got mine via webarchive.
I think
Gamparser can do this too. The dev recently posted on the p99 forums saying he'd fix problems related to p99 for it.
Alternatively like Otikx said you can hit them with a spell and watch how much % of their health is remove. Then do the math. So in his example a 625 dmg nuke took off 25% life. Their life is aprox 2500 in that example.