Parallel acumulator wiring
is 0.3 V dangerous or negligible when wiring them in parallel?


It depends on the internal resistance of the 2 cells you are connecting in parallel. For example, if the 2 cells have a 25mOhm (quite common value for new cells), with a 0.3V difference you get a 6A surge current. This current will quickly drop to zero, though, as the two cells voltages equalize. "Aged" cells have higher internal resistance, and this mitigates the problem. Always try to do your best in equalizing the voltages before connecting the cells in parallel.


  How can I check how many mAh they actually have after I wire them in parallel?


The best way is to discharge the parallel at a constant, known current down to an End of Discharge voltage (usually 2.75-3V for Li-ion), and measuring the discharge time. Then you calculate the capacity as the product of discharge current and discharge time.