Since a while I don't play minecraft but I am sure it is still the same method.
Each mod has its own requisite of the minecraft forge version, but usually all the mods use the same (so I isn't a big problem). Usually (I would say the 99% of times) a newer version of the forge will support mods with a lower requisite. When you download a mod you should do it on pages where it's displayed the Minecraft Forge version requisite like (I think) curseforge or another page.
If you are using a mod which need a newer version of the Minecraft Forge and you load the game it will display a screen that will say which mods aren't compatible and which minimal Forge version you will need.
Instead, if you open your minecraft and get a crash report it will be more difficult to find the propper version. I recommend download the "recommended" version on https://files.minecraftforge.net/ and if that doesn't work you should try the "lastest" version.
Remember, if you aren't sure which is the right version the 90% of times it will be the "recommended" version of https://files.minecraftforge.net/, the 8% of times the "lastest" and the other 2% will be bad luck :(.
Maybe, if you post your minecraft version and which mods you are trying to install I could try it.