When I bought my SLE I had the same question, so I did a lot of research. My conclusion is:
1. If you don't have factory steering wheel controls, you have SLE. All SLTs I've seen have them.
2. If you don't have factory leather seats, you have SLE. All SLTs I've seen have them.
3. Bose is NOT an indicator for whether you have SLE or SLT. There was an option to upgrade to Bose for the SLE. My SLE has the Bose upgrade with the six CD changer.
4. Usually I see SLTs with the pillar logos, but others above me have mentioned that theirs don't so I guess this is inconclusive.
Edmunds reviews are also good sources to get familiar with the differences: 2007 GMC Envoy
Mine now has a Denali front end and I do plan to add leather seating down the road, so I guess it's a hybrid. Only I don't get >15 mpg.