Now you can see what the upgrade will be once you have collected all cards of that type you need in order to buy the upgrade. Is there an in-game way to see what the upgrade will be, so you can choose which type of cards to request / keep?
I know you can check the Clash Royale Wikia, but in-game would save browsing, searching and checking your current level.
Answer
Yes you can. It’s qutie simple actually:
An upgrade adds +10% to the stats.
For troops:
+10% Hit-points
+10% Damage
+10% Damage per Second
For Buildings its:
+10% Hit-points
+1 Troop level/ +10% Damage
For Spells its:
+10% Damage
(for freeze +0.3s duration)
Attribution
Source : Link , Question Author : Jasper de Vries , Answer Author : Mathias711