Go to your profile on Mob Wars. There you should see a tab "fight strength". Click it and you can view the items you'll bring in to fight. On the bottom of the list you'll see the items with lowest attack and defense brought to fight. If those attack or defense is smaller than the items you craft, you'll get stronger. But be careful! Some blueprints are not just for upgrading, some do require several good usable items to craft one just a little better usable item. If the items on blueprint are useless but the craft is usable, good for you!
Nothing to add @The Navigator Just a little thing : sometimes a craft only add +1, or +2, so yes it improves your totals, but it's not worth crafting it.