I believe that at one time Seal offered a type of material that melted at a given temperature. However that is a "go--no go" type assessment and really not indicative of anything other then a threshold temperature.

I have a digital thermometer with thermocouple readings but these are fairly expensive instruments. If you are really interested in obtaining a thermocouple activated digital instrument then I might suggest Ebay (if that is a politically correct mention of names) and search for a thermometer under the business and commercial heading.