How to find the convolution kernel in frequency domain?
If you have noise present in your signal, the straightforward Fourier domain division will cause plenty of errors in your result. Some ways to avoid it are by using the so-called dual channel FFT (Part 1 and Part 2). I can also suggest deconvolution via adaptive filters, LMS or NLMS ([Normalized] Least Mean Squares) filters in particular are easy to understand are not hugely expensive in terms of CPU cycles in case your signals are long. LMS adaptive filters are very robust to noise.