i found when i ref'd the u16 games, you can see the reckless fairly easily, they're at an age where there will be a couple of 'lads' who want to leave a bit on the opposition, or take chunks out of a rival. for example, tackles through the back of the opponent, getting the ball via pushing or pulling an opponent, 'shoulder barges' that floor the opponent because its a "charge" rather than a battle of strength. you can often tell the intention by eyes on the ball or eyes on the player, especially when the balls in the air for the header, watch for those who aren't looking at the ball and are looking to floor the opponent whilst jumping for the header (harry kane got called out for this dangerous behaviour a while back).
also if a player goes to ground for a slide tackle im already thinking any contact with the player its asking for a yellow because there's a lack of control there and a potential to really hurt someone. obviously don't base your decision on what hasn't happened, if they cleanly get the ball then great lets play on. but if an opponent has had to obviously leap out of the way of a slide tackle with a high foot or studs up then im yellow, because im not having a kid going home with a broken leg if i can help it. you will be met with screams of "I DIDNT TOUCH HIM REF" and all you do it go "if he hadn't of jumped you've of wiped him out, its dangerous, stay on your feet where you can."
don't be having any mouth from them, if they get hormonal and narky with each other i used to just go yellow each, don't care who started it, im not having a bad tackle, and im not having retaliation (aggressive attitude), we are here to play football not scrap. and if they carry on chewing my ear off ill tell them i don't care what they see on the telly and have some respect for the game. on that, if there is a coming together of two players and the ball breaks and play continues, keep one eye on the coming together, just takes one little stamp or one shuv for lads that age to completely lose it.
all of this of course is worse case scenario. ive ref'd some lovely u16 games, where they all wanted to play football and there was no bother, the most trouble i got was a few grunts at some decisions they didnt agree with. good luck