Here is one way to do it with the command-line. This information is found in Chapter 1 ("A command-line primer") of the Weka manual that comes with the software.
java weka.classifiers.trees.J48 -t training_data.arff -T test_data.arff -p 1-N
where:
-t <training_data.arff> specifies the training data in ARFF format
-T <test_data.arff> specifies the test data in ARFF format
-p 1-N specifies that you want to output the feature vector and the prediction,
where N is the number of features in your feature vector.
For example, here I am using soybean.arff for both training and testing. There are 35 features in the feature vector:
java weka.classifiers.trees.J48 -t soybean.arff -T soybean.arff -p 1-35
The first few lines of the output look like:
=== Predictions on test data ===
inst# actual predicted error prediction (date,plant-stand,precip,temp,hail,crop-hist,area-damaged,severity,seed-tmt,germination,plant-growth,leaves,leafspots-halo,leafspots-marg,leafspot-size,leaf-shread,leaf-malf,leaf-mild,stem,lodging,stem-cankers,canker-lesion,fruiting-bodies,external-decay,mycelium,int-discolor,sclerotia,fruit-pods,fruit-spots,seed,mold-growth,seed-discolor,seed-size,shriveling,roots)
1 1:diaporth 1:diaporth 0.952 (october,normal,gt-norm,norm,yes,same-lst-yr,low-areas,pot-severe,none,90-100,abnorm,abnorm,absent,dna,dna,absent,absent,absent,abnorm,no,above-sec-nde,brown,present,firm-and-dry,absent,none,absent,norm,dna,norm,absent,absent,norm,absent,norm)
2 1:diaporth 1:diaporth 0.952 (august,normal,gt-norm,norm,yes,same-lst-two-yrs,scattered,severe,fungicide,80-89,abnorm,abnorm,absent,dna,dna,absent,absent,absent,abnorm,yes,above-sec-nde,brown,present,firm-and-dry,absent,none,absent,norm,dna,norm,absent,absent,norm,absent,norm)
The columns are: (1) data instance number; (2) ground truth label; (3) predicted label; (4) error; (5) prediction confidence; and (6) feature vector.