Whether you've been charged with a federal crime can be a surprisingly tricky question.
Sometimes it is obvious. If you were physically arrested by an FBI agent – or other federal law enforcement officer – and brought to a federal district court and presented to a magistrate, then you have definitely been charged. Though you probably already knew that.
Similarly, sometimes you, or your lawyer, will get a letter from a federal prosecutor saying that you've been charged and that you need to come into a federal courthouse for an initial appearance. Often this letter includes a copy of the charging document – an indictment or, more rarely, a criminal complaint – that explains what the charges against you are. Of course, if you or your lawyer receive a letter like this, it means you've been charged. It also means that you've been extended the courtesy of not being dragged out of your home first thing in the morning. Sadly, this courtesy is less frequently extended than it ought to be.
Though sometimes whether you've been charged is less clear.
For example, federal law enforcement may come knocking on your door, ask you some questions, and tell you it is in your best interest to cooperate with them. You'll face a tricky question – whether to talk to them.
It may be that the Secret Service, or some other federal law enforcement, executed a warrant at your office or house. Agents came in and rifled through your papers and personal effects. This can be a deeply disturbing thing – it can shake the way you feel about and interact with the government profoundly. But the execution of a search warrant at your house or business doesn't mean that you've been charged – just that they thought there was evidence at your house and a federal magistrate judge signed off on their request.
Basically, if federal agents came to your door or executed a search warrant at your home or office, all you know is that you're under investigation. But you still don't know if there are federal charges pending against you until you're arrested or told that you have charges pending (in some cases, you may want to work out a plea with the federal prosecutor before you get arrested – though you'll know if you're in that situation).
Finally, and unfortunately, you may have already been charged with a crime and not know it. Federal prosecutors can ask a grand jury to indict you, and then ask a court to seal that indictment. If that happens, you could walk around for days or weeks or months having been charged and not even know it. Though this happens relatively rarely.