if the drain line exists above ground first try forcing a garden hose
up from the exit end. most clogs occur there, right near or at the
exit.

i flush my drain lines at least once a year to prevent clogs.

any chance you could drain the sump directly to the street or wherever
and elminate the pump completely? this is frequently possible and
elminates a big mess of troubles, like pump failure, power failure,
check valve failure, leaking pump lines etc etc,

drain it directly if you can!