I live in an old lake house built about 1938 to 1940 and the entire slab is concrete and stained red, I was told that lead was used in the concrete when the slab was poured. Is this true and how do I cover it up and seal in the lead
Is it true? Try a lead testing kit, commonly available at hardware stores - while primarily intended to test paint, I'd have to think it would give some indication. For a more definitive test you can probably send a sample to a lab. Red Lead was a common pigment, yes. Whether or not that was what was used on your slab, only testing will tell.
Whether you even need to seal in the hypothetical lead is debatable - the primary concerns with lead paint are children eating flaking paint or dust - if the slab does have lead but is not crumbling away and being eaten or inhaled, there isn't a pathway for the hypothetical lead to get into you.
To be safe, just buy an epoxy coat for concrete floors from your home improvement store. It's easy to mix the 2 part epoxy and spread and I've done numerous floors with it. Sika makes a great product and you can use it with or without color flakes. Warning. It will stink pretty bad for a day or two as it cures. Be very careful to start the furthest corner from where you back out the door, and work the room back and forth evenly because you do not want to trap an area that you can't reach or end up walking through it! You will never get it off your shoes.