How do I check if this furnace switch the blower to second speed
Most furnaces; yours included it appears, do not change the speed of the motor during normal operation. The motor typically has three or more speeds (high, medium, and low), and is hard wired to use different speeds based on what mode the furnace is in.  

For example, in heating mode most blowers run at low or medium speed. While in fan or A/C mode, they run at high speed.

If your furnace is overheating, the first thing to do is to make sure the filter(s) are clean. Also make sure all the registers and returns are fully open, and unobstructed.

If you haven't checked already, check the temperature of the furnace when the limit trips. You could have a bad limit, that is tripping when the furnace is not actually overheated.