Hi,
The value perform function returns network performance calculated according to the net.performFcn and net.performParam property values which would differ on the train function you have used. You could check for these property values and get the used error performance metric.
For example
[x,t] = simplefit_dataset;
net = feedforwardnet(20);
net = train(net,x,t);
y = net(x);
perf = perform(net,t,y)
net.performFcn
net.performParam
