How do I check transmission fluid

What is your question?

I had the front differential repaired. Ever since then my Envoy sounds like the transmission is not right. When I first start out it sounds like it is straining and it changes gears differently. Oh, the shop also replaced the front axle at the same time as the differential. Is there a way I could check the transmission fluid and if it is not low, what should I do next? I cannot afford another 600 dollars this month.