Most heatsinks I can find in local stores have absolutely no specifications, so I can have no idea about their K/W values.
Is there a way or good rule of thumb to guess or reliably measure their K/W ratings?
I would find what Farnell sell that looks much like the ones you can buy. If the size and shape of the fins are similar, it must have similar temperature rise.
steady state temperature, surface area, spacing between wings, the colour, spacing in enclosure and ventilation all would influence in the cooling.
my sugestion is to select it by short of trial method if possible.
let it to work and measure teh temperature if it get stable/ steady before the limit of operation then its good, if it still keep on increasing then go for another big size.
I have a transistor with a pot soldered on it's base.
* screw it to the heatsink
* connect a dual-meter PSU set to 10v etc
* turn the pot to set a wattage, like set 1A on the PSU
The wattage drifts a bit as it gets hot so it need to be trimmed a bit when at temp, but it only takes a few mins to measure the heatsink in 'C/W.