A brand new Combro is only 50 quid and is more than adequate. If you can't stump up the cash then you could try your local gunshop who may be able to test, possibly for a fee.
As far as I know there is no accurate way of determining power from looking at penetration on a target. If you're really struggling then a local kind bbs member might step up and lend a hand..?
HW98 .177, Falcon Menace 4.5-18x56, WW tune HW97 blue laminate .177, Hawke eclipse 4-16x50 ir BSA Scorpion se tactical .177 ,BSA deerhunter 6.5-32×46 SMK XS78, .22 3-9x40 Mountmaster- Remington Express xp tactical .22, standard - Gamo CF20 .177 - mk1 meteor .177