Different ExpressJS Error for API vs Page Request?

What is the best way to detect if a request is for an API call versus a page request? I'd like to render a view for page requests and send JSON for API requests. For example:

Common error for page requests:

if (app.get('env') === 'development') {
    app.use(function(err, req, res, next) {
        res.status(err.status || 500);
        res.render('500', {
            message: err.message,
            error: err
        });
    });
}

Common error for API requests:

if (app.get('env') === 'development') {
    app.use(function(err, req, res, next) {
        res.status(err.status || 500);
        res.send({
            message: err.message,
            error: err
        });
    });
}

Thinking about using this but wanted other opinions:

if (app.get('env') === 'development') {
    app.use(function(err, req, res, next) {
        res.status(err.status || 500);
        if (req.url.startsWith('/api')){
            res.send({
                msg: err.message,
                err: err
            });
        } else {
            res.render('500', {
                message: err.message,
                error: err
            });
        }
    });
}