You could use an SPL app on a smartphone to get some idea of what dB your headphones are reaching. Whether that would be any less accurate than a cheap SPL meter, I don't know.
A friend of mine loves loud volume when he uses his headphones (Sennheiser HD25); far greater than anything I can stand.
He reckons he has wrecked his hearing though from many years DJ'ing in a nightclub.
Smartphone apps only tend to really work well when calibrated against an actual meter for reference.