We are running Category 5e cables for IP networking in the same
conduit as 18 awg unshielded twisted pair wire carrying 24 volts.

Two straightforward questions: 

How do I test for electrical interference on the Cat 5e cables?

Based on your experience, how significant of a problem could this be?

Thank you.

As far as interference goes current is more significant than voltage. 
Also the AC component of a signal is more significant than the DC
component as far as interference is concerned.
Do you know the maximum frequency that will be sent through the 18 awg
cable?
Also do you know the likely currents that will travel over these 18
awg cables?
How far are these awg cables from the Cat 5e cables?

My feeling is that the interfrence will be non-existant or very low.

Fluke make several products for testing the quality of cables.
http://www.flukenetworks.com/us/Solutions/Copper+Test+Solutions/Products.htm
However they are expensive.  Probably too expensive to justify for a
one off use like this.

There are likely some cabling firms in our area who do own cable
testing equipment that could do the testing for you.

Another way of determining if any interference is to sent some packets
over you cables and check to see if there is any packet loss.  How you
do this depends on what hardware you are using.

Thank you for the insightful comment.

There is packet loss - pretty significant packet loss - up to 10%
loss. Additionally, this is visually evident as we are running IP
video.

The awg cables are right next to the cat 5e cables in a 3/4" conduit.

When we ran a test of cat 5e cables outside of the conduit, packet
loss dissapeared and the IP video quality dramatically improved.

We feel something is going on with the awg cables but do not know why
or how to test.

Any assistance in answering the question would be greatly appreciated.

the 24 v cables are often used for data and signaling in an industrial
enviroment...  different modes of data transmission are employed
(currrent / pulses /voltage ) .So often it is preferable to have the
network cables run in a seperate conduit. how ever it is often
impractical to run a seperate conduit for a small number of cables.in
such casses it is preferable to have shielded twisted pair cabling
insead of unshielded onesand proper Grounding of the shields is
compulsary. IF it is indeed  a data cable then u will also see
interferencein it aside from seeing it on ur network.
If its not possible to replace that cable with a shielded twisted pair
mayeb u should consider STP cables for ur network
an example of shielded cable for ethernet 
http://216.239.39.100/search?q=cache:lWQA41Dn9m4C:www.a2zcables.com/+shielded+cat+5+cable&hl=en&ie=UTF-8