You will just need to be patient. Once the lemon tree goes through a winter and new growth starts in the spring you may be able to determine that the growth below the graft has leaves that look different than the foliage above. But then again, perhaps this lemon tree was not grafted and the entire tree is okay. A sign of a hostile take over is if the new growth is faster and begins to overpower the top growth. This is your cue to cut out the lower branches trying to pull out the growth eye when you remove the suckers. Keep growing, Marianne Binetti