Extract ticks from plot

You can try to build the automatic ticks manually using the undocumented internal function Charting`FindTicks.

plot = Plot[Sin[x], {x, 0, 20}];

Charting`FindTicks[{0, 1}, {0, 1}] @@ PlotRange[plot][[1]]

{{0., 0}, {5., 5}, {10., 10}, {15., 15}, {20., 20},
 {0., "", {0.005, 0.}, {AbsoluteThickness[0.1]}},
 {1., "", {0.005, 0.}, {AbsoluteThickness[0.1]}},
 {2., "", {0.005, 0.}, {AbsoluteThickness[0.1]}}, 
 .
 .
 .
}

Note that PlotRange[plot] returns the plot range, also undocumented.

Based on the comment of @grbl , there is indeed a workaround to get automatic plot ticks that can be read out using AbsoluteOptions by using CustomTicks. It requires very little extra coding:

plot = Plot[Sin[x], {x, 0, 20}, Ticks -> LinTicks]
xticks = First[Ticks /. First[AbsoluteOptions[plot, Ticks]]]
(*
{{0., " 0", {0.04, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {5., 
  " 5", {0.04, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {10., 
  "10", {0.04, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {15., 
  "15", {0.04, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {20., 
  "20", {0.04, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {1., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {2., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {3., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {4., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {6., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {7., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {8., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {9., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {11., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {12., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {13., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {14., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {16., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {17., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {18., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}, {19., 
  "", {0.015, 0.}, {GrayLevel[0.], AbsoluteThickness[2.]}}}
*)