Soft peaks can be determined by the floppiness of the peaks. If the whisked eggs form peak tops that flop over or melt back down into the peak after pulling up the whisk, then you've reached the "soft peaks" consistency. If no peaks form, then you need to keep whisking to the soft peaks stage. If the peaks that form are firm or even stiff, then you've whisked too far. Be careful though; if the peaks flop back into themselves are grainy in texture, they've been over-whisked; sometimes adding another egg can save the situation but not always.