Excellent welding technique!! How to tell? When welding that combo, and if everything is as it should be....IE: temp, exposure duration, etc., then the line(s) between the 52100 and 420 will be as seen in the photo. IF the temp is too high, or the soak too long......there will be a distinct decarb/transition line, with a crystallized appearance, just below the 420 line. Let that soak in, and then look around the net at these types of blades...... it's a scenario where you mess up intentionally to achieve the best eye appeal......OR you do everything right, have a bit less eye appeal, but have a better cutting implement. Funny how that works isn't it......... give up something, in order the get something. It's a constant theme in Bladesmithing!