I know this question is generally for git, and this question already got answered for git. but I assume some of the users seeing this question are GitHub users so I will add my tip here:

One easy way I found to be somewhat quick and helpful if you use GitHub, is to use GitHub desktop app to see if merging branches will have some conflicts before merge. If you try to merge branches with the GitHub desktop app, it will tell you if there are conflicts BEFORE the merge. It won't tell you how much and in which files though, but in some cases (for me at least) this will be suffice.

Just try to merge and one step before the merge itself it will prompt you that there will be conflicts (it will also tell you if there won't be any conflicts which might be helpful too in some cases)