One possibility is to do what many industries do, use a surrogate or witness sample.  You can either have a companion part that's anodized or otherwise blackened to increase the emissivity or arrange for some part of the workpiece to have a higher emissivity, either through painting or anodizing.  

Otherwise, a surface thermistor may be a another option.

TTFN

The spot radiometer (gun) that most use is typically quite accurate if:
1.  The target size is smaller than the spot(typically written on the side of the gun as size to distance or in the literature- again size to distance.
2.  The background is measured and and your gun is capable of having this inputed to it.
3.  The Emmisivity is measured and inputed to your gun.
4.  The reflectivity is compensated for.

Number 1 above is level 1 thermography, while the remaining 3 are level 2 thermography.

If you are a level 2 thermographer you will know how to determine the above points and decide(typically using the first point) if you can accurately measure your target.

There are many tools (most more expensive) that will help you in making your measurement.  A problem arises when you have not learned how to minimzed the Emmitance/Reflectance/ Transmittance errors in ir readings.  Even the more expensive tools are all but useless if accurate temperature readings are(not delta temperatures) needed and the thermographer does not know how to minimize ERT errors.

If you are attempting to observe the welding process, odds are great that the spot at the weld is too small for any gun.  If you are looking for the area around the weld as it is accomplished, odds are great that with experience and the use of the same type materials and process you will be able to determine poor welding work.  Otherwise, you can expect to require an ir imager. The imagers available simply have a much smaller spot size to distance than a typical ir gun.

Please respond if you wish to provide more details into your specific requirement.  We may be able to point you towards additional procedures and courses that might fill in the blanks.

By the way, I have shot butt welding in progress for two steel tubing manufacturing plants.

Good luck in your efforts.