Some easy things to check to make sure your icon set feels consistent:
- Corner radius
- Line treatment (stroke weight; cap treatment; ect)
- Overall weight (ie. if each pixel was a drop of paint, would they all make more or less the same tone when you mix them together?)
- Use of colour
- Are they all tilted (or not tilted) at the same angle?
- Do they handle connecting pieces similarly (either by leaving spaces or by finding ways to connect multiple objects)?
- Do they have either relatively similar silhouettes (eg. all contained in circles) or relatively unique silhouettes?