Three options....
1 - stall without feeding to see if the number rises in 30 minutes or so on its own
2 - give what is called a "BCS" dose, which is a lower dose if you aren't comfortable giving the normal 1.0u dose. This doesn't not mean, however, that you would necessarily see "less" effect from the insulin. The effects of an increase or a decrease are not "immediate" in many cases due to the depot nature of Lantus.
3 - shoot the full dose. Provided you have plenty of test strips, food in LC, HC and MC varieties, and can get tests at +1 and +2 for starters. Tests after that would depend on what you see at those tests.
The other option is skipping the shot altogether, but I have the feeling that you'd prefer not to do that?