Since onion 'grass' is an Allium (A. vineale) too you'd need to be familiar with the foliage of whatever species of intentional Allium you planted in order to tell the difference. If you post pictures of what you have here and on the Name That Plant Forum perhaps we can help. One feature to look for is whether the leaves are flat or cylindrical. Onion Grass would be cylindrical. Many garden Alliums are flat. But that's not universal.