I would say that the differences are vague, but exist.
A.) Consults will usually be for a condition that is not known (definitively) at the time of the request; in addition, I would expect that the consultant would offer opinion or advice and the overall treatment of the condition would not be established at the time of consult request. Let me go on to say that consultants do not always assume care of the patients' condition, but rather provide suggestions to the primary, attending team.
B.) 90801 (Psych Evals) are very comprehensive services. You can think of them as a History and Physical for mental status. There will be many, many items covered in a psych eval, which would include items on psychiatric history or status, social information, behavioral evaluations. Psych evals may result in testing and we'd expect to see some level of assumption of care for the patients' mental conditions.
I realize that my summary is not comprehensive, nor is it representative of all considerations. Coders seem to struggle with mitigating between the consultation and psych eval codes--I don't blame them. It is extremely difficult. There may be tools that folks can suggest on the psych forum here, so be sure you review any of that information.
Good luck to you.
Kevin B. Shields, RHIT, CPCO, CCS, CPC, COC, CCS-P, CPC-P, CPC-I