I am still laughing,but I'll attempt to answer your question:
You could investigate by using the scientific hypothesis method by disproving the hypothesis that there was NOT anal entry of any kind.
Start with:
Looking for hairs on the bed, collect any that look like they don't belong to you or her.
Taking a cotton swab sample of the anal sample space region while in the doggy style position. You could briefly stick your finger in her anus and then swipe it on the cotton swab. Be advised that the cotton swab may be contaminated by your finger. The more accurate method would be to take a plastic baggy and a cotton swab and directly apply it to the anus; immediately after dispose of the cotton swab in the plastic baggy, without contaminating it. Confuse your girlfriend by telling her you found pathological piles around her anus and that the haemorrhoids look acute and need treatment. Tell your girlfriend you are worried about her. Be prepared for riding either the a.angry dragon or the b.immediate seizure of evidence.
a. If a. occurs enjoy the ride
b. If b. occurs try to hide the evidence in your own anus or just throw it under the bed.
Send the sample, along with a piece of DNA of your own and your girlfriends DNA (hair?fingernail clippings?Look around the toilet seat for hair and the garbage can in the washroom for nail clippings), to a DNA testing facility, such as:
http://www.easydna.ca/Legal-dna-test.html
Be prepared to spend up to $300
This could lead to several discoveries:
A.Feces, yes shit...your girlfriend might have taken one not too long ago. Hence the gaping anus, but I think you will find shit residue regardless of whether a shit was taken recently or not. Therefore, this neither proves nor disproves your hypothesis, because. If only two types of DNA are found you are safe...she must indeed have taken a shit or shoved something up her anus shortly before having sex with you.
***I have actually been suspicious once or twice about why my gf's ass was gaping while I was doing her from behind. I just tried to stick it in there and she said: "aaaaaah no please don't" I dismissed my suspicions immediately, but in your case you may have anal sex frequently. Thus, she may have motive!!!
B. Three different types of DNA are found. Someone was riding her Hershey highway, you were right afterall! Now you need to build your case carefully, you wouldn't want to have it thrown out because of technicalities.
*If you want to find out who was in there I suggest sending in those bed hairs, if you found any, and also take hair samples from your friends, aunts, uncles, brother, sisters, fathers, mothers...anybody could be to blame for the gaping. Remember if you look long and hard enough you will find the evidence you seek! Plant it if you have to;
You Might base your divorce settlement on adultery, who knows!