A few simple methods can help.

• Mirror trick: Place a small mirror flat against the wall/subject. When the camera is exactly perpendicular to that surface, you’ll see the camera reflected straight back. For best accuracy, make sure the centre of the lens lines up with the centre of the mirror/frame using your camera’s center focus point or grid.
• Use frame edges and grid lines: Zoom or compose so the wall edges, floor/ceiling lines, or other straight features sit near the edges of the viewfinder/screen. Adjust until the perspective looks even on both sides and top/bottom.
• Use natural reference lines: In interiors, align with floorboards, tiles, furniture edges, or other lines known to be parallel/perpendicular to the walls.
• Lock your position and recompose carefully: You can crouch or angle the camera briefly to check alignment, then return to your shooting position while keeping the camera orientation unchanged.
• Correct in post if needed: Software such as Lightroom can correct some vertical and horizontal perspective errors, though it’s not as good as getting it right in-camera or using a tilt-shift lens.