How do i know if my pain on left side is from colon or siatica?

Different radiation: Pain caused by abdominal problems is usually dull and aching and does not radiate (spread) down the leg. Sciatic pain usually follows the distribution of the sciatic nerve. It may be associated with activities and can be sharp, burning or tingling. No matter what the the cause of the pain is if it lasts two or more weeks it needs to be investigated. Don't try to figure it out by yourself.

Need MRI: If you are concerned, i would consider asking your physician to order you an MRI to evaluate your spine.