No there really aren't signs that tell you that. You can get an idea of the traffic level by the pricing. It's dynamic and goes up as the traffic increases so if you see a fare close to $1/mile you know traffic is heavy. Lowest you will see I believe is $.20/mile. Btw, the real backup going south is where the HOT lanes merge into the normal I-95 lanes around Stafford. Mile + backups in the HOT lanes are not uncommon during busy times, there are complaints on a regular basis about spending the $ for HOT but then getting stuck in traffic at the end.
I'd be surprised if you need the HOT lanes on Thanksgiving Day. Before you hit the Beltway, you might check Googlemaps with the traffic filter turned on to see if there's any yellow or red.
Have a safe trip!