I believe the Gospel writers openly say when Jesus is giving a parable. Parables, by definition in our day, and in Jesus’ day, are fictional stories used to illustrate religious truths. Here is a good example from Matthew 13:3-15.
And he spoke to them at length in parables, saying: “A sower went out to sow. And as he sowed, some seed fell on the path, and birds came and ate it up. Some fell on rocky ground, where it had little soil. It sprang up at once because the soil was not deep, and when the sun rose it was scorched, and it withered for lack of roots. Some seed fell among thorns, and the thorns grew up and choked it. But some seed fell on rich soil, and produced fruit, a hundred or sixty or thirtyfold. Whoever has ears ought to hear.”
The disciples approached him and said, “Why do you speak to them in parables?” He said to them in reply, "Because knowledge of the mysteries of the kingdom of heaven has been granted to you, but to them it has not been granted. To anyone who has, more will be given and he will grow rich; from anyone who has not, even what he has will be taken away. This is why I speak to them in parables, because ‘they look but do not see and hear but do not listen or understand.’ Isaiah’s prophecy is fulfilled in them, which says: ‘You shall indeed hear but not understand you shall indeed look but never see. Gross is the heart of this people, they will hardly hear with their ears, they have closed their eyes, lest they see with their eyes and hear with their ears and understand with their heart and be converted, and I heal them.’
Notice that he usually explains his parables to the disciples in private when they don’t understand.
When Jesus talks about the Eucharist in John 6 and in the last supper narratives in Matthew, Mark, and Luke, there is no indication that he is speaking to them in parables. In fact, had he been speaking in a parable for John 6, the people would not have left him because they would have known he was only speaking metaphorically. He would also have clarified his point to the disciples in private, had he been speaking metaphorically, but he does not.
Hope this helps.