Is there a way to find my REM sleep?

Well this could be useful for me if i knew how my sleep cycles worked.
I know the largest REM sleep starts after 4-6 hours of sleep generally, but i can't
tell which is the most appropriate time to wake up in a middle of a REM sleep for me.
This could be useful to do FILD or WBTB. Generally i put my alarm clock after 4 hours
but i don't feel any difference in my dreams vividness.

I have an app called "Android Sleep" and it monitors your movement in sleep to determine your sleep cycles. you can also set an alarm for a certain time and it will wake you up RIGHT before a REM cycle :D

Although it's only a 2-week free trial, I think it would be worth it to buy the whole thing if it will wake you up in REM sleep every time you set the alarm (it's made to wake you up before a REM cycle so you can go to work or school more refreshed).