The NFL's current regulations on gloves state that "adhesive or slippery substances on the body, equipment, or uniform of any player" are not permitted. This includes gloveless hands. Illegal gloved tackles happen from time to time, but they are rare. If you make a questionable call on whether or not you saw a player with gloveless hands, then you have more concern about yourself than about the rules. The referees are trained observers who can tell if a player is wearing gloves by their shape and size on the body. They also can see the condition of a player's hands under the skin before making a call on a fumble. Gloveless tackling was once common in college football, but that has changed over time. There were even calls for it to be banned because of its impact on the game. However, researchers found that players tended to avoid gloveless tackles if they knew what kind of attention they would get from officials. This shows that illegal gloved tackles are controlled by fans rather than risk getting penalized themselves. Overall, the chances of you being involved in an illegal gloved tackle are very low. If it does happen, don't worry about it too much - focus instead on your own play.