Usually I have a training partner so it's not an issue, but occasionally I require someone else in the gym to spot me on the bench press.
I require that person to help me unrack the bar and rack it at the end of the set, not to help me complete a rep.
After asking for a spot, I usually say, "Just help me unrack and rack, don't make a rep easier for me", but every time when they see me slowing down through the last few reps, I feel them deload the weight. Then as I'm 'thanking' them, I have to guess whether I would have completed the set by myself or not.
Is there a better way? I was slowly but surely getting to the top of a shoulder press the other day when someone walked past and helped, so I'm thinking this could be tricky.