Good question. Ideally, you'd have a schematic and parts list to work off of.
Otherwise, you can go by the type of construction used and size. Here's a comparison of some common types.
Left to right we have 1/2 W, 1W and 2W carbon composition. Last it's a vitreous enamel 10W power resistor. Generally resistors over 2 watts are marked with a wattage rating. Hope that helps.
