How can you tell the difference between a cysts or lipoma? I have a marble sized lump under skin. Easily moves around with little pressure No pain.

Transillumination. One way to determine if a subcutaneous mass is fluid filled (cyst) or more solid (lipoma) is to use a small penlight or similar. While in a darkened room you place the light source as close as possible to the cyst from one side or the other to see whether the light transilluminates the mass. If little light is visible across the mass, it is likely more solid. Positive light indicates fluidic cyst.
Consistency locatio. Cyst: fluid filled and Lipoma, Soft tissue solid filled.
Cyst versus Lipoma. It can be difficult to tell the difference between a fluid filled cyst and a soft benign fatty lipoma if you are not a doctor. Both lesions present as soft well circumscribed lesions that are not painful and can be easily compressed. A fluid filled cyst demonstrates water properties...if you tap it you can elicit a fluid wave. Lipomas can be large and arise from subcutaneous fat under the skin.