I'm Huddersfield Town and before the first season began I got Michael Mifsud for free. He finished the season scoring 47 goals in 49 games. Most valuable signing which was good. He got 38 in the league and 9 in cup games. Quality signing.
Also got Luis Boa Morte on a free during second season. Played half the season then stopped performing really so managed to sell him for £1.5m
I find that the best way is offer a one week trial to loads of them or scout them if possible. You'll find that when they arrive on trial, the report given by your asst manager is quite a good indication. Look at their history too to see how they performed in previous season or if they were unattached then.
Also people, I forgot that I signed Javan Vidal in my second season. He's young and it sometimes shows but I'd say he's been my overperforming free agent this season. Very happy signing, especially as I may sell him to higher club soon with a buy back price in the agreement