Try looking at other maps of the same song and analyze their timing. (If they exist). If not, you can also sometimes Google a songs BPM and if it's popular enough. You might get a relatively accurate result. But it's not perfect and a lot of the time can be wrong. But it's normally a good starting point. BPM calculation softwares also suffer the same issue. So I don't really recommend using any.
Timing is something that seems complicated but personally I think becomes very easy. (At least with most basic songs). Songs will normally be a round number. No decimal points. (This does happen occasionally but isn't very common). A good way to do thing is tap along to the beat with T. This is normally a good start to judging a songs BPM. Normally you wont get it perfectly by manually tapping. But I use it as a starting point, as I now know it's only a few BPM off. You can either just start changing the number up and or down by one, saving and then seeing. Or you can listen for when it starts to fall off and become un-synchronized. and adjust the BPM then. It gets pretty simple once you start fiddling around with it.