Are you cutting into a 2x6 wall or a 2x4 wall? It's not always the case, but the majority of the time an interior load bearing wall will be built with 2x6. After that, if there is any way for you to determine which way the floor joists run, where the span of those joists end/begin and where any beams may be located, it would be an easy decision. I realize that might be a tough thing to find out though. Also, is it a newer home or older? If it's really old you might not be able to, but if it's newer you could contact the builder and they could probably supply you with the floor plan/joist layout.
Is your basement finished? If not, you can go into the basement and easily determine where all bearing points are. Well, at least if it was done properly and all bearing points were carried to the foundation, which they are supposed to be. One last thing, if you do determine it's load-bearing, you can still do the reno, it just has to be done so that you frame the opening in properly, meaning you'd have to header it off with 2x10's or microlam while properly supporting it in the meantime.
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