The OSHA regulation does not specify who is to conduct the check. It would be advisable to contact the OSHA local area office for guidance on the qualifications needed. Also, the manufacturer’s instructions should be followed.
In addition, NFPA 10, Standard for Portable Fire Extinguishers, (which OSHA references in 29 CFR 1910 Subpart L, Appendix B) may provide useful information. This standard provides detail about who can inspect and service equipment. It requires, among other things, that “maintenance, servicing, and recharging shall be performed by trained and certified persons having available the appropriate servicing manual(s) ...”
In addition, it is always a good idea to check with the local government fire authority for any additional requirements.