Most reliable way to decode PAM2 signal with unknown signal levels?
I suggest as a simple approach start with the threshold you chose and then bin the samples that are above and below threshold and take the average of each of those separately and then adjust your threshold to be between those two values. Mid way would be optimum when the two symbols are equiprobable in the presence of identical balanced noise on all samples (meaning the distribution going positive is the same as the distribution going negative). In the case on more 1’s than 0’s or vice versa with known or computed SNR, the threshold decision would be determined by a weighted average of the two symbol means using the SNR’s to be closer to the symbol that occurs less likely. (To optimize bit error rate). 

Given the OP's comment under the question of using complex IQ samples with Gaussian noise, the 0 bit would be Rayleigh distributed and the 1 bit would be Ricean. The approach above could determine the estimated variance for each of the symbols (and therefore SNR from mean and variance) and then if the probability of occurrence is known for each symbol, a threshold can be determined that would minimize the error rate by equating the net areas of each distribution beyond the threshold weighted by the probability of occurrence.