Irregular period:? Need a few answers idk anymore

I don't know how to word this. But I was on depo for about a year n a half (I think) once I met my fiancé (2013) I stopped going.. It says it takes 18 months to leave system, I've read anyway.. Depo messed up my periods bad and made me gain a lot of weight..now lately my periods of come around the 7-10th.. (Past few months) but before April they came at end of month.. Now I don't know if their regular or not. I'm gonna wait till the 10th to see if AF shows. But I guess my question is how would you know if theirs a chance of you being  pregnant if you don't know when you'd get your period?? 
And or ...Anyone with irregular periods what was your symptoms and how far along were you when you found out? Just kinda want answers. Been to doctors.. Supposed to get my blood drawn when AF shows to check my hormones and see if any things wrong.. Than have to have my fiancé checked to see if it could possibly be him.. He is 10 years older than me.. And has been through a lot Lol so.. Sorry this is so long.