How do you know when you're in REM?

For WBTB or a nap during the day, how can you tell if you are in REM?

You will be in a dream.

you can't tell while you're sleeping (unless you become lucid). i always know when i wake up if i've just been in REM or if it's been a while. if you JUST woke up from a dream then you were in REM. if the dream is fuzzy, like when you wait a few minutes after waking to try and remember it, then you were not in REM. it's complicated. you just kind of know, you know?

I mean like. during the day how can you tell when you can go into REM?

i think it's anywhere between when you wake up and 9 hours after. idk. you'll have to look it up on google or something. i wake up usually at 5 am and i can get into REM during a nap anytime before 7 pm, so i guess everyone is different.