The story of how the Imperial Stormtroopers™ managed to track down down the droids is told in a booklet that was packaged with the Imperial Troop Transporter™ toy.
In short, the Stormtoopers identified tracks from both droids leading away from the pod in different directions. They used their transporter (a repulsorlift craft that leaves no tracks) to trace one set and the other set was followed by a Dewback™ unit. The set followed by the transporter led them directly to the Jawa Sandcrawler™.
I've reproduced the story in its entirety below.
It seemed impossible! Two mindless
droids had made off with design prints of the
Empire’s most devastating weapon. ..Death
As the search squad left the Imperial
Cruiser, a report came through from the on-
board Scanning Stations. The droids’ Escape
Pod had been tracked to an arid region on
Tatooine known as the Central Lowlands.
The exact ground position had been lost as
the Pod went beyond the Scanner’s range,
but, in all that sand, a metal Pod should not
be hard to find, not with the Imperial Troop
Landing at pre-set coordinates, the Troop
Transporter of the Imperial Empire was there to
meet them...long, sleek, deadly, with the Imperial Crest over the side hatches. The great, steel
blue craft was operated by two drivers and could
hold all six members of the search squad. Each
Trooper entered a Traveling Rack on the side of
the Transporter. They rode standing up in individual racks since their white and black armor
made it almost impossible to sit. The rear of the
vehicle contained a Prisoner Immobilization
Unit for the detention and taming of renegade
droids. And, for heavy resistance, there was
always the Laser Blaster mounted on the upper
deck. Even at night, the Transporter looked fearsome with the glare of its black, flip-up lights.
One driver reported that Dewback™ ground
units had spotted the Pod just a short distance to
the north. Not bad for a Dewback outfit! But,
the Transporter would take over the search from
that point. It could cover twice the distance in
half the time.
The Transporter hovered a few feet above the
surface and sped to the Pod landing site, but the
droids and the plans had long since gone. They
left only two sets of tracks in the sand that were
spared by the mid-day winds. The Dewback had
already gone after one set of tracks. The Transporter locked onto the other set and headed east.
The Surface Scanner in the Transporter soon
picked up an old Class “D” transport slogging
through the sand. Something the Jawas called
a Sandcrawler™ Over the external sound system, they were ordered to halt in the name of the
Empire. They kept moving. Two rounds were
fired across the Sandcrawler bow from the 50
megatome Blaster on the upper deck. The Jawas
stopped and came out to greet the Imperial
They had many droids in their inventory, but
not the ones Darth Vader wanted back. Two
droids were looted from the cargo bay and
placed in the Droid Prisoner Compartment for
further “questioning". A sale had been recorded
to one Owen Skywalker of two droids for his
moisture farm. That’s all the Troop Commander
had to hear. The order was given to open fire and
the Jawas were cut down in mid-step...
As the Transporter left to find the Skywalker
farm, the heavy gun fired once at the
Sandcrawler’s mid-section. It exploded and
burst into flames.