What would be the best internal control for a small business with limited number of staff and segregation of duty is not practical?

continuous monitoring OR random checks by the management

periodic audits

Direct supervision from the owner on all critical business activities acts as a compensatory control when adequate segregation of duties are not possible. Moreover, if resources permit, it may be considered to appoint an internal auditor on a contract basis to further support the owner to maintain or strengthen the control over business.