How to inspect gain of a biomedical signal?

We have recorded an emg signal using an amplifier and the gain of amplifier was set to 10 thousand , and we know that the amplitude of EMG signal is between 0-10mv . Now I want to inspect the amplitude of signal. Like if the minimum amplitude of signal was 1 mv before amplification then after an amplification of 10,000 , that value should be 10mv. but when i plot the signal after loading it into matlab, I am unable to find any sign of gain because the amplitude lies between +-0.25. I am attaching one channels data