Each account has an ECOA Code which designates the account type as either sole, joint, maker (i.e., co-signor), or authorized user.
Yes, a party reviewing your credit report can tell if there are any AU accounts being included.
AU accounts are not free lunch.
If an AU account is included in your scoring, yes, you can get a score boost, but the score is NOT then representative of your own personal credit risk. Lendors are aware of that fact, and it may effect their view of your credit score in their determination process.
Typically, adding an AU account that benefits your scoring generally also benefit approval chances if the amount of the new credit is relatively low. Many creditors offering lower amounts of credit limit dont invest in a more detailed and expensive manual review of your entire credit report, and thus are not aware whether the score they are considering is based in part on the credit history of another.
Thus, adding an AU to your report/scoring is most often beneficial when you are building or rebuilding.
However, as you move up the credit ladder and seek higher levels of credit or credit limits, the chance of lendor manual review of your entire credit report increases, and they will likely see that one or more accounts are AU's, and may choose to place less reliance on a score they know is not representative of only your own personal credit risk. It is common, for example, for some mortgage lendors to require the removal of AUs as part of their underwriting process so that they can get a "real" score that assesses your true repayment risk.
I would generally go for the AU, but you may wish to later remove it when your score begins to approach 700'ish, and/or you are seeking a high principal chunk of credit, such as a mortgage or auto loan.
