I would try to dissassemble it enough to see the other side of that. It may be only a visual damage on paint,Does this represent a crack/frame damage? Is the bike safe to ride?
This is like taking your clothing off so the doctor can examine the patient.Remove paint around crack. Chip off with hammer and screwdriver or cold chisel. Clean it up with wire brush, dremel tool or what ever you got. Find the fattest heaviest person. Look at donut shops. Have him/her sit on seat and bounce up and down. Inspect crack with flashlight and strong magnifying glass. Eat more donuts.
Got no experience with this crack finding dye. Before you buy. Examine like I said. "Inspect crack with flashlight and strong magnifying glass." If you see crack. No need to buy nothing. If crack is not visible? DO buy some crack finding dye.Could be a poor job de-greasing the weld prior to painting, but i would strip back down and sand off the surrounding paint and spray with some crack finding dye.. Google Search
(sorry for the long link, edit tools are unavailable to embed under text atm?)