The best way is an infrared thermometer, sorry. You can get them for $25. They (the $25 ones) are well accurate enough for meaningful data. I finally bought one, and it has been indispensible since. Makes quick temp checks so easy, even on the animals themselves w/o disturbing them. Much more meaningful data than even just the basking spot.
Other than that, those stick-on aquarium thermometers will give you an idea, but you have to lay them on the tile and allow a few minutes before reading. They are not as accurate but are good enough to tell you if the tile is too hot.
My advice to you is this - if you plan on keeping herps from now on, get an infrared thermometer. There really is no better tool for this application.
-----
Mark