How to show paidby and check_number on Contribution - Receipt (on-line)
I figured out myself.

Put this API call somewhere upfront in the template. This retrieve contribution payment method by ID. (Get the whole record if removing "return" parameter.)

     {crmAPI var='apiResult' entity='Contribution' action='get' return="payment_instrument_id" contribution_id=$contributionID}


At where it should be displayed:

     {foreach from=$apiResult.values item=contribution}

         {$contribution.payment_instrument}

     {/foreach}