Mana spike?
When his soldiers appear and start blocking ants, I'm pretty sure there's a surge in mana consumption. So at the very least Jinwoo feels that and understands the situation is getting dire when that surge happens.
Also, it is possible he leveled up his ability to communicate and get data from his long distance shadows between his first teleport and now.
But basically even without levelling up those parts, mana draw is a very basic yet easy to use method.