Alternate signer

There is no way that I can think of.  There are at least two free credit score services that I could find that have score simulators, that will allow you to estimate your credit score if you closed or paid off an account.  The problem is that if you removed yourself as an authorized signer, you could then ask the credit bureaus to remove the accounts completely from your records, and having those accounts not listed at all is different than listing them as closed or paid off.

Generally speaking, if the accounts are paid on time and do not have any late payments or past due amounts, then they probably help more than they hurt. They increase the amount of credit available to you, and the increase the average length of your credit history, particularly if your relative has had these accounts for a long time.  If the accounts are nearly maxed out, this will have a negative affect, and any late payments or past due amount will certainly have a large negative affect on your score.

But I don’t know of any credit score simulator that would allow you to remove an account as if it never existed. They would show you the effect of closing the account or paying down the balance.