Raul,
How hard is it for you to remove the L297 from the circuit? I assume you have checked power and gnd to the L298? If you can easily remove the L297, I would suggest putting a pair of load resistors to simulate the motor. Test one output by a voltmeter across the load from out1 to out 2. With ena low, there should be no voltage reading (<.2v) from out1 to out2 regardless of the state of in1 and in2. When in1 and in2 are in oposite logic states you should read your power supply voltage minus a couple of volts, reversing the opposite states on in1 and in2 should reverse the polarity of the voltage reading. For example if you motor supply voltage is 20, in one state I would expect approximately 18V the opposite state it would switch to -18V.
Phil
p.s. we have a single sided pcb available here:
layout:
http://webpages.charter.net/pminmo/L297_8board.pdf
Schematic:
http://webpages.charter.net/pminmo/L297_8schematic.pdf
Copper for pcb:
http://webpages.charter.net/pminmo/L297_8copper.pdf
The pcb is for toner transfer home brew pcb's