Programmatically with many expressions you can try this:
TraditionalForm[HoldForm[Det[{{a, 8}, {b, 1}}]]]
TraditionalForm[Defer[Det[{{a, 8}, {b, 1}}]]]

Read help on HoldForm and Defer and also take a look at Inactive which is useful in other situations. Also this blog could help:
Converting to Conventional Mathematical Typesetting