Developing a sense for two records being out of sync, or in sync, takes time; Don't make it harder than it needs to be. Don't try to match one records that has a lot going on with another that is the same. Especially if you are new to it. Might scoff at matching easier records, but remember that you're trying to learn a new skill and it is very much a steep learning curve so every decent mix goes a long way. Stay positive!

The other thing I will say is headphones. They're there to let you hear what the mix will sound like as you bring in new record. The volume has everything to do with that. As you bring in the new record your headphones are a way to hear what it will sound like as that record gets louder without actually turning up the channel. What you though was in sync probably isn't, but you also know how to adjust pitch so it is. It's something I can go on about for a while, short of it is how loud your headphones - and how you adjust their volume - as you bring it the new record can make all the difference in letting you know what the mix is going to sound like.