This app was mentioned in 7 comments, with an average of 2.29 upvotes
Someone created a website you can check, also includes past week's rewards for reference.
There is also some phone apps and normally on Friday you will see a few topics regarding his inventory.
If you dont want to miss him next time, try something like this app.
I found this helpful and simple. Gives a push notification when he arrives (fridays), you can look up his location and the weekly items, and it will also warn you a few hours before he leaves (tuesdays).
I would highly recommend the Android app. It shows what he's selling and user submitted ratings for each piece.
To be honest. I appreciate the person who created the app more https://play.google.com/store/apps/details?id=com.crocusgames.whereisxur
there is a great app on google play and ios called where is xur? that has a voting system so you can vote what you want him to have and hes been correct for the past 6 months. Free as well https://play.google.com/store/apps/details?id=com.crocusgames.whereisxur&hl=en