Hi Doni,
There are a number of ways to spot bad track,
1) look for bends or warping of the rails, you can spot these by setting the piece of track on a flat surface, if you can rock it either from end to end or diagonally that is a bad piece.
2) on straight track look down at the length of the rail for dents or kinks and for any bends to left or right.
3) look at the ends of the track to see if there may be a broken rail joining pin or pins that are badly bent, when you try to straighten them they may break
4) look for excessive rust especially on the top of the rail, a light bit of rust on the side of the rail is not bad
5) look for ties that are crushed, bent, or flattened out, look also at the ties where the rail is attached for missing insulators
These are a few tips, hope this helps you
Ray