What are the signs that baby is going to crawl or pull up ? Is this when you drop the crib mattress?

We have a very big baby (99 percentile) who’s very active. He’s 5 months on Tuesday. He rolls all over, on his tummy pulls his knees up under himself and last night I saw him push up from the tummy down knees under position to a straight elbows with only hands and knees on the floor. It was only for a second but he’s been very motivated to teach himself things.

In the early mornings sometimes we let him babble and roll around in the crib for a few mins if he’s happy. I don’t want to get caught off guard with moving the mattress down and since hubby and I both work long hrs. There’s is a very very small window on weeknights to do any projects before baby goes to bed (in the crib). So it’s probably a weekend project.

Thanks!!