Many localities have free utility marking services for homeowners who need to dig in their yard.
If you cannot, or don't want to, drill your own test hole, then there are likely monitoring wells nearby that could give you a good idea.
Most local governments have some sort of Department of Groundwater Resources or Well water services etc. that maintain information such as ground water level.
With a little research, I think that you should be able to find someone who could help. When you contact someone, if they say that they don't know, ask them if they know who would know. There is almost definitely someone who has this information.
The Department of Public Works or the Health department might be able to help or direct you to someone who can.
https://www.twdb.state.tx.us/groundwater/data/gwdbrpt.asp
http://www.waterdatafortexas.org/groundwater/about
http://dallascityhall.com/trinity_watershed/index.html
The USGS maintains information that might be helpful. If they don't have the information, they could probably advise you on where to get the information, or who to contact.
https://water.usgs.gov/ogw/data.html
http://tx.usgs.gov/
Pool builders, or other builders who dig, might have the information you need. The builder who built your pool should know if they hit water. They might have put in a well point under ground for water control.
A local residential well driller might be able to help. They could even drill the test hole for you. Companies that do foundation repair could probably help. A professional geotechnical engineering services company could advise you.
There is usually a stream, pond etc. close enough to help give you an idea. It can be man-made, or natural.
