Okay so let me tell you something 
I went through this exact situation and I wish someone had made a thread like this earlier. So here is what I actually did, step by step, no cap.
First thing I tried was just making a new Tinder account. I used a spare Gmail and a Google Voice number because I did not want to burn my real number on this. Set my location to the neighborhood we both live in, set the age range to match his, set my preference to men, and just started swiping.
Now here is the thing nobody tells you: you have to be patient. The free version only gives you so many likes per day before it locks you out. So I swiped for like 20 minutes every evening for about a week. On day four, I found his profile. Same photos he had on Instagram. Bio was different though, newer and updated, which told me it was not an old account he forgot to delete. That stung.
The reason I am sharing this is because it actually works but it is also mentally exhausting. Every time I opened the app I felt anxious. So my advice: if you are going to do this method, decide upfront what you will do with the information once you have it. Have a plan. Because finding out is just step one, the conversation after is the real thing you have to prepare for.
Also: if you find nothing after a solid week of searching, that is actually a real result too. Do not keep going forever. At some point you have to decide to either talk to your partner or trust them. The app can only tell you so much 