You'll want to use a Sails policy for this:
// /api/policies/isMobile.js
module.exports = function(req, res, next) {
var ua = req.headers['user-agent'];
req.isMobile = /mobile/i.test(ua);
next();
}
// /config/policies.js
module.exports.policies = {
'*': 'isMobile'
};
This will run the code before every controller action, and give you access to the req.isMobile var in all of your custom controller code.