Chain slipping

Reconstituted mtb with new chain on used 9s cassette & 3x rings. (Bought the
bike used and never rode it before now so don't know its distant history.)

Standing on pedals in high gear (big-small -- 44 & 11) is OK except for once
every few revolutions of the pedals when the chain slips a tooth.

Rear derailleur is installed, front is not. (Have no shifters installed yet;
this was just a validation ride to check frame, suspension, steering, brakes,
bearings, etc.)

How do I determine if front or rear gears are the problem?

Turning pedals backward shows no stiff links in the chain.

Thanks.