It depends if this is part of your usual job or not? If it is, then the answer is no you can't claim CPEs. If it isn't, then the answer is probably, yes you can.
If the standard has your name in the credits then I'd say that's good proof. Otherwise keep a track of what you do and when. Keep copies of any outputs. Keep copies of any correspondence. Take photos of you participating, etc.
Ultimately, you need to provide enough proof to convince an auditor, should you be audited, that you completed the activity you're claiming the CPEs for.
Just think if you were that auditor, what would you accept as evidence?
https://www.isc2.org/cpe-handbook
EDIT: I should also add, if this work is covered under the domains of your certification then you can claim group A CPEs, otherwise they will be group B.
