We used a machine learning (ML) approach to detect bulbar amyotrophic lateral sclerosis (ALS) prior to the onset of overt speech symptoms. The dataset included speech samples from 123 participants who were stratified by sex and into three groups: healthy controls, ALS symptomatic, and ALS presymptomatic. We compared models trained on three group pairs (symptomatic-control, presymptomatic-control, and all ALS-control participants). Using acoustic features obtained with the OpenSMILE ComParE13 configuration, we tested several feature filtering techniques. ML classification was achieved using an SVM model and leave-one-out crossvalidation. The most successful model, which was trained on symptomatic-control data, yielded an AUC=0.99 for females and AUC=0.91 for males. Models trained on all ALS-control participants had high diagnostic accuracy for classifying symptomatic and presymptomatic ALS participants (females: AUC=0.85; males: AUC=0.91). Additionally, probabilities from these models correlated with speaking rate (females: Spearman coefficient=-0.60, p<0.001; males: Spearman coefficient=-0.43, p<0.001) and intelligible speaking rate (females: Spearman coefficient=-0.65, p<0.001; males: Spearman coefficient=-0.40, p<0.01), indicating their possible use as a severity index of bulbar motor involvement in ALS. These results highlight the importance of stratifying patients by speech severity when testing diagnostic models and demonstrate the potential of ML classification in early detection and progress monitoring of ALS.