Justin Bieber -- Telltale Tagging Reveals Signs of Drug Abuse

Justin Bieber's problem with sizzurp had early warning signs ... but no one picked up on them.

We went back and looked at this photo ... taken last month. You see a skate ramp in Justin's backyard, covered in spray paint.

Check out the reference to "double cup," with a styrofoam cup next to it. Double cup is code for lean mixed with double the amount of codeine.

As we first reported ... we've learned Bieber has been deep into the lean game lately -- it's codeine and promethazine mixed with Sprite and a Jolly Rancher. Bieber's also been abusing Rx drugs, as well as booze and weed.