How do I know if…

How can I find out if my identity has been stolen/compromised? A website like Credit Karma doesn’t show anything specific, nothing suspicious, but I tried to file my taxes through H&R Block and they said it wouldn’t go through because something has been compromised, but they couldn’t be more specific?!? I got in touch with FTC and I’m waiting to hear back… what else can I do to find out more?